private function _call_add_order($add_order, $transaction_id)
 {
     try {
         $wsdl_url = $this->config->item('wsdl_url');
         $client = new SoapClient($wsdl_url);
         $response = $client->AddOrder(array('AddOrderRequest' => $add_order));
         $result = $response->AddOrderResult;
         if ($result->Ack == 'Success') {
             return $result->TrackCode;
         } else {
             if (isset($result->Message)) {
                 $this->epacket_model->save_failure_message($transaction_id, $result->Message);
             }
             return false;
         }
     } catch (SOAPFault $exception) {
         ob_start();
         print $exception;
         $error_message = ob_get_contents();
         ob_end_clean();
         $this->epacket_model->save_failure_message($transaction_id, $error_message);
     }
 }