diff --git a/QuickPay/API/Client.php b/QuickPay/API/Client.php
index cc420e7..25f5d41 100644
--- a/QuickPay/API/Client.php
+++ b/QuickPay/API/Client.php
@@ -10,13 +10,21 @@ class Client
     public CurlHandle $ch;
     protected ?string $auth_string;
     protected array $headers = [];
+    protected string $api_url = "";
 
-    public function __construct(?string $auth_string = '', array $additional_headers = [])
+    public function __construct(?string $auth_string = '', array $additional_headers = [], $api_url = Constants::API_URL)
     {
         if (!function_exists('curl_init')) {
             throw new GenericException('Lib cURL must be enabled on the server');
         }
 
+        $this->api_url = $api_url;
+        if($api_url == Constants::INVOICING_API_URL){
+            // no idea why, but invoicing requires this header.
+            $additional_headers = array_merge($additional_headers,['Accept: application/vnd.api+json']);
+        }
+
+
         // Save authentication string
         $this->auth_string = $auth_string;
 
@@ -40,6 +48,10 @@ public function __construct(?string $auth_string = '', array $additional_headers
         $this->setHeaders($additional_headers);
     }
 
+    function getApiURL(){
+        return $this->api_url;
+    }
+
     public function shutdown(): void
     {
         if (!empty($this->ch)) {
diff --git a/QuickPay/API/Constants.php b/QuickPay/API/Constants.php
index 4fa5f76..d0b60c2 100644
--- a/QuickPay/API/Constants.php
+++ b/QuickPay/API/Constants.php
@@ -4,6 +4,7 @@
 
 class Constants
 {
-    public const API_URL     = 'https://api.quickpay.net/';
-    public const API_VERSION = '10';
+    public const API_URL                = 'https://api.quickpay.net/';
+    public const INVOICING_API_URL      = 'https://invoicing.quickpay.net/';
+    public const API_VERSION            = '10';
 }
diff --git a/QuickPay/API/Request.php b/QuickPay/API/Request.php
index 9282a7f..25ce5bc 100644
--- a/QuickPay/API/Request.php
+++ b/QuickPay/API/Request.php
@@ -60,7 +60,7 @@ public function delete(string $path, array $form = []): Response
 
     protected function setUrl(string $url): void
     {
-        curl_setopt($this->client->ch, CURLOPT_URL, Constants::API_URL . trim($url, '/'));
+        curl_setopt($this->client->ch, CURLOPT_URL, $this->client->getApiURL() . trim($url, '/'));
     }
 
     protected function execute(string $request_type, array $form = []): Response
diff --git a/QuickPay/QuickPay.php b/QuickPay/QuickPay.php
index 208659a..99b4b45 100644
--- a/QuickPay/QuickPay.php
+++ b/QuickPay/QuickPay.php
@@ -13,9 +13,9 @@ class QuickPay
 
     public Request $request;
 
-    public function __construct(string $auth_string = '', array $additional_headers = [])
+    public function __construct(string $auth_string = '', array $additional_headers = [], $api_url = Constants::API_URL)
     {
-        $client        = new Client($auth_string, $additional_headers);
+        $client        = new Client($auth_string, $additional_headers, $api_url);
         $this->request = new Request($client);
     }