diff --git a/Models/Invoice.php b/Models/Invoice.php
index f645cf2..e4b41e6 100644
--- a/Models/Invoice.php
+++ b/Models/Invoice.php
@@ -180,31 +180,79 @@ class Invoice implements \JsonSerializable
         return $this->id;
     }
 
+    /**
+     * Get invoice number.
+     *
+     * @return string
+     *
+     * @since 1.0.0
+     */
     public function getNumber() : string
     {
         return $this->number;
     }
 
+    /**
+     * Set invoice number.
+     *
+     * @param string $number Invoice number
+     *
+     * @return void
+     *
+     * @since 1.0.0
+     */
     public function setNumber(string $number) : void
     {
         $this->number = $number;
     }
 
+    /**
+     * Get type
+     *
+     * @return int
+     *
+     * @since 1.0.0
+     */
     public function getType() : int
     {
         return $this->type;
     }
 
+    /**
+     * Set type
+     *
+     * @param int $type Type
+     *
+     * @return void
+     *
+     * @since 1.0.0
+     */
     public function setType(int $type) : void
     {
         $this->type = $type;
     }
 
+    /**
+     * Get status
+     *
+     * @return int
+     *
+     * @since 1.0.0
+     */
     public function getStatus() : int
     {
         return $this->status;
     }
 
+    /**
+     * Set status
+     *
+     * @param int $status Status
+     *
+     * @return void
+     *
+     * @since 1.0.0
+     */
     public function setStatus(int $status) : void
     {
         $this->status = $status;
@@ -222,12 +270,28 @@ class Invoice implements \JsonSerializable
         return $this->createdAt;
     }
 
+    /**
+     * Set shipping date.
+     *
+     * @param \DateTime $send Shipping date
+     *
+     * @return void
+     *
+     * @since 1.0.0
+     */
     public function setSend(\DateTime $send) : void
     {
         $this->send = $send;
     }
 
-    public function getSend()
+    /**
+     * Get shipping date.
+     *
+     * @return null|\DateTime
+     *
+     * @since 1.0.0
+     */
+    public function getSend() : ?\DateTime
     {
         return $this->send;
     }
@@ -244,306 +308,795 @@ class Invoice implements \JsonSerializable
         return $this->createdBy;
     }
 
+    /**
+     * Set creator.
+     *
+     * @param int $creator Creator
+     *
+     * @return void
+     *
+     * @since 1.0.0
+     */
     public function setCreatedBy(int $creator) : void
     {
         $this->createdBy = $creator;
     }
 
+    /**
+     * Get client.
+     *
+     * @return mixed
+     *
+     * @since 1.0.0
+     */
     public function getClient()
     {
         return $this->client;
     }
 
+    /**
+     * Set client.
+     *
+     * @param mixed $client client
+     *
+     * @return void
+     *
+     * @since 1.0.0
+     */
     public function setClient($client) : void
     {
         $this->client = $client;
     }
 
+    /**
+     * Set shipping receiver.
+     *
+     * @param string $ship Shipping receiver
+     *
+     * @return void
+     *
+     * @since 1.0.0
+     */
     public function setShipTo(string $ship) : void
     {
         $this->shipTo = $ship;
     }
 
+    /**
+     * Get shipping receiver.
+     *
+     * @return string
+     *
+     * @since 1.0.0
+     */
     public function getShipTo() : string
     {
         return $this->shipTo;
     }
 
+    /**
+     * Set shipping fao.
+     *
+     * @param string $ship FAO
+     *
+     * @return void
+     *
+     * @since 1.0.0
+     */
     public function setShipFAO(string $ship) : void
     {
         $this->shipFAO = $ship;
     }
 
+    /**
+     * Get shipping fao.
+     *
+     * @return string
+     *
+     * @since 1.0.0
+     */
     public function getShipFAO() : string
     {
         return $this->shipFAO;
     }
 
+    /**
+     * Set shipping address.
+     *
+     * @param string $ship Shipping address
+     *
+     * @return void
+     *
+     * @since 1.0.0
+     */
     public function setShipAddress(string $ship) : void
     {
         $this->shipAddress = $ship;
     }
 
+    /**
+     * Get shipping address.
+     *
+     * @return string
+     *
+     * @since 1.0.0
+     */
     public function getShipAddress() : string
     {
         return $this->shipAddress;
     }
 
+    /**
+     * Set shipping city.
+     *
+     * @param string $ship City
+     *
+     * @return void
+     *
+     * @since 1.0.0
+     */
     public function setShipCity(string $ship) : void
     {
         $this->shipCity = $ship;
     }
 
+    /**
+     * Get shipping city.
+     *
+     * @return string
+     *
+     * @since 1.0.0
+     */
     public function getShipCity() : string
     {
         return $this->shipCity;
     }
 
-    public function setShipZip($ship) : void
+    /**
+     * Set shipping zip.
+     *
+     * @param string $ship Zip
+     *
+     * @return void
+     *
+     * @since 1.0.0
+     */
+    public function setShipZip(string $ship) : void
     {
         $this->shipZip = $ship;
     }
 
+    /**
+     * Get shipping zip.
+     *
+     * @return string
+     *
+     * @since 1.0.0
+     */
     public function getShipZip() : string
     {
         return $this->shipZip;
     }
 
+    /**
+     * Set shipping country.
+     *
+     * @param string $ship Country
+     *
+     * @return void
+     *
+     * @since 1.0.0
+     */
     public function setShipCountry(string $ship) : void
     {
         $this->shipCountry = $ship;
     }
 
+    /**
+     * Get shipping country.
+     *
+     * @return string
+     *
+     * @since 1.0.0
+     */
     public function getShipCountry() : string
     {
         return $this->shipCountry;
     }
 
+    /**
+     * Set billing receiver.
+     *
+     * @param string $bill Billing receiver
+     *
+     * @return void
+     *
+     * @since 1.0.0
+     */
     public function setBillTo(string $bill) : void
     {
         $this->billTo = $bill;
     }
 
+    /**
+     * Get billing receiver.
+     *
+     * @return string
+     *
+     * @since 1.0.0
+     */
     public function getBillTo() : string
     {
         return $this->billTo;
     }
 
+    /**
+     * Set billing fao.
+     *
+     * @param string $bill FAO
+     *
+     * @return void
+     *
+     * @since 1.0.0
+     */
     public function setBillFAO(string $bill) : void
     {
         $this->billFAO = $bill;
     }
 
+    /**
+     * Get billing fao.
+     *
+     * @return string
+     *
+     * @since 1.0.0
+     */
     public function getBillFAO() : string
     {
         return $this->billFAO;
     }
 
+    /**
+     * Set billing address.
+     *
+     * @param string $bill Billing address
+     *
+     * @return void
+     *
+     * @since 1.0.0
+     */
     public function setBillAddress(string $bill) : void
     {
         $this->billAddress = $bill;
     }
 
+    /**
+     * Get billing address.
+     *
+     * @return string
+     *
+     * @since 1.0.0
+     */
     public function getBillAddress() : string
     {
         return $this->billAddress;
     }
 
+    /**
+     * Set billing city.
+     *
+     * @param string $bill City
+     *
+     * @return void
+     *
+     * @since 1.0.0
+     */
     public function setBillCity(string $bill) : void
     {
         $this->billCity = $bill;
     }
 
+    /**
+     * Get billing city.
+     *
+     * @return string
+     *
+     * @since 1.0.0
+     */
     public function getBillCity() : string
     {
         return $this->billCity;
     }
 
-    public function setBillZip($bill) : void
+    /**
+     * Set billing zip.
+     *
+     * @param string $bill Zip
+     *
+     * @return void
+     *
+     * @since 1.0.0
+     */
+    public function setBillZip(string $bill) : void
     {
         $this->billZip = $bill;
     }
 
+    /**
+     * Get billing zip.
+     *
+     * @return string
+     *
+     * @since 1.0.0
+     */
     public function getBillZip() : string
     {
         return $this->billZip;
     }
 
+    /**
+     * Set billing country.
+     *
+     * @param string $bill Country
+     *
+     * @return void
+     *
+     * @since 1.0.0
+     */
     public function setBillCountry(string $bill) : void
     {
         $this->billCountry = $bill;
     }
 
+    /**
+     * Get billing country.
+     *
+     * @return string
+     *
+     * @since 1.0.0
+     */
     public function getBillCountry() : string
     {
         return $this->billCountry;
     }
 
+    /**
+     * Set referer.
+     *
+     * @param int $referer Referer
+     *
+     * @return void
+     *
+     * @since 1.0.0
+     */
     public function setReferer(int $referer) : void
     {
         $this->referer = $referer;
     }
 
+    /**
+     * Get referer.
+     *
+     * @return int
+     *
+     * @since 1.0.0
+     */
     public function getReferer() : int
     {
         return $this->referer;
     }
 
+    /**
+     * Set referer name.
+     *
+     * @param string $refererName Referer name
+     *
+     * @return void
+     *
+     * @since 1.0.0
+     */
     public function setRefererName(string $refererName) : void
     {
         $this->refererName = $refererName;
     }
 
+    /**
+     * Get referer name.
+     *
+     * @return string
+     *
+     * @since 1.0.0
+     */
     public function getRefererName() : string
     {
         return $this->refererName;
     }
 
+    /**
+     * Set tax id.
+     *
+     * @param string $tax Tax id
+     *
+     * @return void
+     *
+     * @since 1.0.0
+     */
     public function setTaxId(string $tax) : void
     {
         $this->taxId = $tax;
     }
 
+    /**
+     * Get tax id.
+     *
+     * @return string
+     *
+     * @since 1.0.0
+     */
     public function getTaxId() : string
     {
         return $this->taxId;
     }
 
+    /**
+     * Set insurance.
+     *
+     * @param Money $insurance Insurance fee
+     *
+     * @return void
+     *
+     * @since 1.0.0
+     */
     public function setInsurance(Money $insurance) : void
     {
         $this->insurance = $insurance;
     }
 
+    /**
+     * Get insurance.
+     *
+     * @return Money
+     *
+     * @since 1.0.0
+     */
     public function getInsurance() : Money
     {
         return $this->insurance;
     }
 
+    /**
+     * Set freight.
+     *
+     * @param Money $freight Freight fee
+     *
+     * @return void
+     *
+     * @since 1.0.0
+     */
     public function setFreight(Money $freight) : void
     {
         $this->freight = $freight;
     }
 
+    /**
+     * Get freight.
+     *
+     * @return Money
+     *
+     * @since 1.0.0
+     */
     public function getFreight() : Money
     {
         return $this->freight;
     }
 
+    /**
+     * Get net amount.
+     *
+     * @return Money
+     *
+     * @since 1.0.0
+     */
     public function getNet() : Money
     {
         return $this->net;
     }
 
+    /**
+     * Get gross amount.
+     *
+     * @return Money
+     *
+     * @since 1.0.0
+     */
     public function getGross() : Money
     {
         return $this->gross;
     }
 
+    /**
+     * Set currency.
+     *
+     * @param string $currency Currency
+     *
+     * @return void
+     *
+     * @since 1.0.0
+     */
     public function setCurrency(string $currency) : void
     {
         $this->currency = $currency;
     }
 
+    /**
+     * Get currency.
+     *
+     * @return string
+     *
+     * @since 1.0.0
+     */
     public function getCurrency() : string
     {
         return $this->currency;
     }
 
+    /**
+     * Set info.
+     *
+     * @param string $info Info
+     *
+     * @return void
+     *
+     * @since 1.0.0
+     */
     public function setInfo(string $info) : void
     {
         $this->info = $info;
     }
 
+    /**
+     * Get info.
+     *
+     * @return string
+     *
+     * @since 1.0.0
+     */
     public function getInfo() : string
     {
         return $this->info;
     }
 
+    /**
+     * Set payment term.
+     *
+     * @param int $payment Payment term
+     *
+     * @return void
+     *
+     * @since 1.0.0
+     */
     public function setPayment(int $payment) : void
     {
         $this->payment = $payment;
     }
 
-    public function getPayment()
+    /**
+     * Get payment term.
+     *
+     * @return null|int
+     *
+     * @since 1.0.0
+     */
+    public function getPayment() : ?int
     {
         return $this->payment;
     }
 
+    /**
+     * Set payment text.
+     *
+     * @param string $payment Payment text
+     *
+     * @return void
+     *
+     * @since 1.0.0
+     */
     public function setPaymentText(string $payment) : void
     {
         $this->paymentText = $payment;
     }
 
+    /**
+     * Get payment text.
+     *
+     * @return string
+     *
+     * @since 1.0.0
+     */
     public function getPaymentText() : string
     {
         return $this->paymentText;
     }
 
+    /**
+     * Set shipping terms (e.g. inco).
+     *
+     * @param int $terms Terms
+     *
+     * @return void
+     *
+     * @since 1.0.0
+     */
     public function setTerms(int $terms) : void
     {
         $this->terms = $terms;
     }
 
-    public function getTerms()
+    /**
+     * Get terms.
+     *
+     * @return int
+     *
+     * @since 1.0.0
+     */
+    public function getTerms() : ?int
     {
         return $this->terms;
     }
 
+    /**
+     * Set terms text.
+     *
+     * @param string $terms Terms text
+     *
+     * @return void
+     *
+     * @since 1.0.0
+     */
     public function setTermsText(string $terms) : void
     {
         $this->termsText = $terms;
     }
 
+    /**
+     * Get terms text.
+     *
+     * @return string
+     *
+     * @since 1.0.0
+     */
     public function getTermsText() : string
     {
         return $this->termsText;
     }
 
+    /**
+     * Set shipping.
+     *
+     * @param int $shipping Shipping (e.g. incoterm)
+     *
+     * @return void
+     *
+     * @since 1.0.0
+     */
     public function setShipping(int $shipping) : void
     {
         $this->shipping = $shipping;
     }
 
-    public function getShipping()
+    /**
+     * Get shipping.
+     *
+     * @return int
+     *
+     * @since 1.0.0
+     */
+    public function getShipping() : ?int
     {
         return $this->shipping;
     }
 
+    /**
+     * Set shipping text.
+     *
+     * @param string $shipping Shipping text
+     *
+     * @return void
+     *
+     * @since 1.0.0
+     */
     public function setShippingText(string $shipping) : void
     {
         $this->shippingText = $shipping;
     }
 
+    /**
+     * Get shipping text.
+     *
+     * @return string
+     *
+     * @since 1.0.0
+     */
     public function getShippingText() : string
     {
         return $this->shippingText;
     }
 
+    /**
+     * Get vouchers.
+     *
+     * @return array
+     *
+     * @since 1.0.0
+     */
     public function getVouchers() : array
     {
         return $this->vouchers;
     }
 
+    /**
+     * Add voucher.
+     *
+     * @param string $voucher Voucher code
+     *
+     * @return void
+     *
+     * @since 1.0.0
+     */
     public function addVoucher(string $voucher) : void
     {
         $this->vouchers[] = $voucher;
     }
 
+    /**
+     * Get tracking ids for shipment.
+     *
+     * @return array
+     *
+     * @since 1.0.0
+     */
     public function getTrackings() : array
     {
         return $this->trackings;
     }
 
+    /**
+     * Add tracking id.
+     *
+     * @param string $tracking Tracking id
+     *
+     * @return void
+     *
+     * @todo: do same for actual tracking information per trackign id
+     *
+     * @since 1.0.0
+     */
     public function addTracking(string $tracking) : void
     {
         $this->trackings[] = $tracking;
     }
 
+    /**
+     * Get invoice elements.
+     *
+     * @return array
+     *
+     * @since 1.0.0
+     */
     public function getElements() : array
     {
         return $this->elements;
     }
 
+    /**
+     * Add invoice element.
+     *
+     * @param mixed $element Invoice element
+     *
+     * @return void
+     *
+     * @since 1.0.0
+     */
     public function addElement($element) : void
     {
         $this->elements[] = $element;
@@ -552,8 +1105,8 @@ class Invoice implements \JsonSerializable
     /**
      * {@inheritdoc}
      */
-    public function jsonSerialize() : void
+    public function jsonSerialize()
     {
-
+        return [];
     }
 }
diff --git a/Models/InvoiceElement.php b/Models/InvoiceElement.php
index 08b1700..97591ae 100644
--- a/Models/InvoiceElement.php
+++ b/Models/InvoiceElement.php
@@ -54,8 +54,6 @@ class InvoiceElement implements \JsonSerializable
 
     private $singleDiscountR = 0;
 
-    private $totalDiscountR = null;
-
     private $discountQ = 0;
 
     private $singlePriceNet = null;
@@ -64,7 +62,7 @@ class InvoiceElement implements \JsonSerializable
 
     private $taxP = null;
 
-    private $taxR = null;
+    private $taxR = 0.0;
 
     private $singlePriceGross = null;
 
@@ -76,16 +74,6 @@ class InvoiceElement implements \JsonSerializable
 
     private $invoice = 0;
 
-    /**
-     * Constructor.
-     *
-     * @since 1.0.0
-     */
-    public function __construct()
-    {
-
-    }
-
     /**
      * Get id.
      *
@@ -98,197 +86,484 @@ class InvoiceElement implements \JsonSerializable
         return $this->id;
     }
 
+    /**
+     * Set event.
+     *
+     * @param int $event Event
+     *
+     * @return void
+     *
+     * @since 1.0.0
+     */
     public function setEvent(int $event) : void
     {
         $this->event = $event;
     }
 
+    /**
+     * Get event.
+     *
+     * @return mixed
+     *
+     * @since 1.0.0
+     */
     public function getEvent()
     {
         return $this->event;
     }
 
+    /**
+     * Set promotion.
+     *
+     * @param int $promotion Promotion
+     *
+     * @return void
+     *
+     * @since 1.0.0
+     */
     public function setPromotion(int $promotion) : void
     {
         $this->promotion = $promotion;
     }
 
+    /**
+     * Get promotion.
+     *
+     * @return string
+     *
+     * @since 1.0.0
+     */
     public function getPromotion()
     {
         return $this->promotion;
     }
 
+    /**
+     * Set order.
+     *
+     * @param int $order Order
+     *
+     * @return void
+     *
+     * @since 1.0.0
+     */
     public function setOrder(int $order) : void
     {
         $this->order = $order;
     }
 
-    public function getorder() : int
+    /**
+     * Get order.
+     *
+     * @return int
+     *
+     * @since 1.0.0
+     */
+    public function getOrder() : int
     {
         return $this->order;
     }
 
+    /**
+     * Set item.
+     *
+     * @param mixed $item Item
+     *
+     * @return void
+     *
+     * @since 1.0.0
+     */
     public function setItem($item) : void
     {
         $this->item = $item;
     }
 
+    /**
+     * Set item name.
+     *
+     * @param string $name Name
+     *
+     * @return void
+     *
+     * @since 1.0.0
+     */
     public function setItemName(string $name) : void
     {
         $this->itemName = $name;
     }
 
+    /**
+     * Get item name.
+     *
+     * @return string
+     *
+     * @since 1.0.0
+     */
     public function getItemName() : string
     {
         return $this->itemName;
     }
 
+    /**
+     * Set item description.
+     *
+     * @param string $description Description
+     *
+     * @return void
+     *
+     * @since 1.0.0
+     */
     public function setItemDescription(string $description) : void
     {
         $this->itemDescription = $description;
     }
 
+    /**
+     * Get item description.
+     *
+     * @return string
+     *
+     * @since 1.0.0
+     */
     public function getItemDescripion() : string
     {
         return $this->itemDescription;
     }
 
-    public function setQuantity(int $quantity) : void
+    /**
+     * Set quantity.
+     *
+     * @param int|float $quantity Quantity
+     *
+     * @return void
+     *
+     * @since 1.0.0
+     */
+    public function setQuantity($quantity) : void
     {
         $this->quantity = $quantity;
     }
 
-    public function getQuantity() : int
+    /**
+     * Get quantity.
+     *
+     * @return int|float
+     *
+     * @since 1.0.0
+     */
+    public function getQuantity()
     {
         return $this->quantity;
     }
 
+    /**
+     * Set single unit price.
+     *
+     * @param Money $price Single unit price
+     *
+     * @return void
+     *
+     * @since 1.0.0
+     */
     public function setSinglePrice(Money $price) : void
     {
         $this->singlePrice = $price;
     }
 
-    public function getSinglePrice() : Money
+    /**
+     * Get single unit price.
+     *
+     * @return null|Money
+     *
+     * @since 1.0.0
+     */
+    public function getSinglePrice() : ?Money
     {
         return $this->singlePrice;
     }
 
+    /**
+     * Set total price.
+     *
+     * @param Money $price Total price
+     *
+     * @return void
+     *
+     * @since 1.0.0
+     */
     public function setTotalPrice(Money $price) : void
     {
         $this->totalPrice = $price;
     }
 
-    public function getTotalPrice() : Money
+    /**
+     * Get total price.
+     *
+     * @return null|Money
+     *
+     * @since 1.0.0
+     */
+    public function getTotalPrice() : ?Money
     {
         return $this->totalPrice;
     }
 
+    /**
+     * Set discount price.
+     *
+     * @param Money $discount Discount
+     *
+     * @return void
+     *
+     * @since 1.0.0
+     */
     public function setDiscountPrice(Money $discount) : void
     {
         $this->singleDiscountP = $discount;
     }
 
-    public function getDiscountPrice() : Money
+    /**
+     * Get discount price.
+     *
+     * @return null|Money
+     *
+     * @since 1.0.0
+     */
+    public function getDiscountPrice() : ?Money
     {
         return $this->singleDiscountP;
     }
 
+    /**
+     * Set total discount price.
+     *
+     * @param Money $discount Discount price
+     *
+     * @return void
+     *
+     * @since 1.0.0
+     */
     public function setTotalDiscountPrice(Money $discount) : void
     {
         $this->totalDiscountP = $discount;
     }
 
-    public function getTotalDiscountPrice() : Money
+    /**
+     * Get total discount price.
+     *
+     * @return null|Money
+     *
+     * @since 1.0.0
+     */
+    public function getTotalDiscountPrice() : ?Money
     {
         return $this->totalDiscountP;
     }
 
+    /**
+     * Set discount percentage.
+     *
+     * @param float $discount Discount percentag
+     *
+     * @return void
+     *
+     * @since 1.0.0
+     */
     public function setDiscountPercentage(float $discount) : void
     {
         $this->singleDiscountR = $discount;
     }
 
+    /**
+     * Get discount percentage.
+     *
+     * @return float
+     *
+     * @since 1.0.0
+     */
     public function getDiscountPercentage() : float
     {
         return $this->singleDiscountR;
     }
 
-    public function setTotalDiscountPercentage(float $discount) : void
-    {
-        $this->totalDiscountR = $discount;
-    }
-
-    public function getTotalDiscountPercentage() : float
-    {
-        return $this->totalDiscountR;
-    }
-
+    /**
+     * Set discount quantity.
+     *
+     * @param int|float $quantity Quantity
+     *
+     * @return void
+     *
+     * @since 1.0.0
+     */
     public function setDiscountQuantity($quantity) : void
     {
         $this->discountQ = $quantity;
     }
 
+    /**
+     * Get discount quantity.
+     *
+     * @return int|float
+     *
+     * @since 1.0.0
+     */
     public function getDiscountQuantity()
     {
         return $this->discountQ;
     }
 
+    /**
+     * Set single net price.
+     *
+     * @param Money $price Net price
+     *
+     * @return void
+     *
+     * @since 1.0.0
+     */
     public function setSingleNetPrice(Money $price) : void
     {
         $this->singlePriceNet = $price;
     }
 
-    public function getSingleNetPrice() : Money
+    /**
+     * Get single net price.
+     *
+     * @return null|Money
+     *
+     * @since 1.0.0
+     */
+    public function getSingleNetPrice() : ?Money
     {
         return $this->singlePriceNet;
     }
 
+    /**
+     * Set total net price.
+     *
+     * @param Money $price Total net price
+     *
+     * @return void
+     *
+     * @since 1.0.0
+     */
     public function setTotalNetPrice(Money $price) : void
     {
         $this->totalPriceNet = $price;
     }
 
-    public function getTotalNetPrice() : Money
+    /**
+     * Get total net price.
+     *
+     * @return null|Money
+     *
+     * @since 1.0.0
+     */
+    public function getTotalNetPrice() : ?Money
     {
         return $this->totalPriceNet;
     }
 
+    /**
+     * Set tax price.
+     *
+     * @param Money $tax Tax price
+     *
+     * @return void
+     *
+     * @since 1.0.0
+     */
     public function setTaxPrice(Money $tax) : void
     {
         $this->taxP = $tax;
     }
 
-    public function getTaxPrice() : Money
+    /**
+     * Get tax price.
+     *
+     * @return null|Money
+     *
+     * @since 1.0.0
+     */
+    public function getTaxPrice() : ?Money
     {
         return $this->taxP;
     }
 
-    public function setTaxPercentag(float $tax) : void
+    /**
+     * Set tax rate.
+     *
+     * @param float $tax Tax rate
+     *
+     * @return void
+     *
+     * @since 1.0.0
+     */
+    public function setTaxPercentage(float $tax) : void
     {
         $this->taxR = $tax;
     }
 
-    public function getTaxPercentag() : float
+    /**
+     * Get tax rate.
+     *
+     * @return float
+     *
+     * @since 1.0.0
+     */
+    public function getTaxPercentage() : float
     {
         return $this->taxR;
     }
 
+    /**
+     * Set single gross price.
+     *
+     * @param Money $price Single gross price
+     *
+     * @return void
+     *
+     * @since 1.0.0
+     */
     public function setSingleGrossPrice(Money $price) : void
     {
         $this->singlePriceGross = $price;
     }
 
-    public function getSingleGrossPrice() : Money
+    /**
+     * Get single gross price.
+     *
+     * @return null|Money
+     *
+     * @since 1.0.0
+     */
+    public function getSingleGrossPrice() : ?Money
     {
         return $this->singlePriceGross;
     }
 
+    /**
+     * Set total gross price.
+     *
+     * @param Money $price Total gross price
+     *
+     * @return void
+     *
+     * @since 1.0.0
+     */
     public function setTotalGrossPrice(Money $price) : void
     {
         $this->totalPriceGross = $price;
     }
 
-    public function getTotalGrossPrice() : Money
+    /**
+     * Get total gross price.
+     *
+     * @return null|Money
+     *
+     * @since 1.0.0
+     */
+    public function getTotalGrossPrice() : ?Money
     {
         return $this->totalPriceGross;
     }
@@ -296,8 +571,8 @@ class InvoiceElement implements \JsonSerializable
     /**
      * {@inheritdoc}
      */
-    public function jsonSerialize() : void
+    public function jsonSerialize()
     {
-
+        return [];
     }
 }