public function sendRequest($endpoint, $content, $method = 'POST')
 {
     $token = hash_hmac('sha256', json_encode($content), $this->config['API_SECRET_KEY']);
     // Set authorization header
     $headers = array('X-Account-ID: ' . $this->config['ACCOUNT_ID'], 'X-API-Key: ' . $this->config['API_KEY'], 'X-API-Token: ' . $token);
     switch (strtolower($method)) {
         case 'post':
             $response = Request::curlPost(self::API_URL . $endpoint, $headers, $content);
             break;
         case 'get':
             $response = Request::curlGet(self::API_URL . $endpoint, $headers, $content);
             break;
     }
     return new BillingAppResponse($response);
 }