diff --git a/QuickPay/API/Client.php b/QuickPay/API/Client.php
index 3cad955..8a17980 100644
--- a/QuickPay/API/Client.php
+++ b/QuickPay/API/Client.php
@@ -18,6 +18,13 @@ class Client
      */
     public $ch;
 
+    /**
+     * Base url for the selected API.
+     *
+     * @var string
+     */
+    public $base_url;
+
     /**
      * Contains the authentication string
      *
@@ -31,8 +38,11 @@ class Client
      * Instantiate object
      *
      * @access public
+     * @param string $auth_string   Format 'username:password' or ':apiKey'
+     * @param string $base_url      The API to call. Use on of the constants.
+     * @throws Exception
      */
-    public function __construct($auth_string = '')
+    public function __construct($auth_string = '', $base_url = Constants::API_URL)
     {
         // Check if lib cURL is enabled
         if (!function_exists('curl_init')) {
@@ -42,6 +52,9 @@ public function __construct($auth_string = '')
         // Set auth string property
         $this->auth_string = $auth_string;
 
+        // Set base url of selected API
+        $this->base_url =  $base_url;
+
         // Instantiate cURL object
         $this->authenticate();
     }
@@ -71,10 +84,20 @@ protected function authenticate()
     {
         $this->ch = curl_init();
 
-        $headers = array(
-            'Accept-Version: v10',
-            'Accept: application/json',
-        );
+        $headers = array();
+        switch ($this->base_url) {
+            case Constants::API_URL_INVOICING:
+                $headers[] = 'Accept: application/vnd.api+json';
+                break;
+
+            case Constants::API_URL:
+                $headers[] = 'Accept-Version: v' . Constants::API_VERSION;
+                $headers[] = 'Accept: application/json';
+                break;
+
+            default:
+                break;
+        }
 
         if (!empty($this->auth_string)) {
             $headers[] = 'Authorization: Basic ' . base64_encode($this->auth_string);
diff --git a/QuickPay/API/Constants.php b/QuickPay/API/Constants.php
index ecbe5b2..4e60325 100644
--- a/QuickPay/API/Constants.php
+++ b/QuickPay/API/Constants.php
@@ -12,8 +12,13 @@
 class Constants
 {
     /**
-     * API DEFINITIONS
+     * Primary API
      */
     const API_URL = 'https://api.quickpay.net/';
     const API_VERSION = '10';
+
+    /**
+     * Invoicing API
+     */
+    const API_URL_INVOICING = 'https://invoicing.quickpay.net/';
 }
diff --git a/QuickPay/API/Request.php b/QuickPay/API/Request.php
index 9c40ee9..094f43b 100644
--- a/QuickPay/API/Request.php
+++ b/QuickPay/API/Request.php
@@ -18,6 +18,7 @@ class Request
      * Contains QuickPay_Client instance
      *
      * @access protected
+     * @var Client
      */
     protected $client;
 
@@ -139,7 +140,7 @@ public function delete($path, $form = array())
      */
     protected function setUrl($params)
     {
-        curl_setopt($this->client->ch, CURLOPT_URL, Constants::API_URL . trim($params, '/'));
+        curl_setopt($this->client->ch, CURLOPT_URL, $this->client->base_url . trim($params, '/'));
     }
 
     /**
diff --git a/QuickPay/QuickPay.php b/QuickPay/QuickPay.php
index 98ba3f3..8c0f24f 100644
--- a/QuickPay/QuickPay.php
+++ b/QuickPay/QuickPay.php
@@ -14,18 +14,19 @@ class QuickPay
     public $request;
 
     /**
-    * __construct function.
-    *
-    * Instantiates the main class.
-    * Creates a client which is passed to the request construct.
-    *
-    * @auth_string string Authentication string for QuickPay
-    *
-    * @access public
-    */
-    public function __construct($auth_string = '')
+     * __construct function.
+     *
+     * Instantiates the main class.
+     * Creates a client which is passed to the request construct.
+     *
+     * @param string $auth_string    Authentication string for QuickPay. Format 'username:password' or ':apiKey'
+     * @param string $base_url       Optional: Use a secondary API (eg billing)
+     *
+     * @access public
+     */
+    public function __construct($auth_string = '', $base_url = \QuickPay\API\Constants::API_URL)
     {
-        $client = new Client($auth_string);
+        $client = new Client($auth_string, $base_url);
         $this->request = new Request($client);
     }
 }