private function getIPNIdentifierFromOrder(TdbShopOrder $oOrder)
    {
        $sIdentifier = null;
        $query = "SELECT `shop_payment_handler_group`.*
                    FROM `shop_payment_handler_group`
              INNER JOIN `shop_payment_handler` on `shop_payment_handler_group`.`id` = `shop_payment_handler`.`shop_payment_handler_group_id`
              INNER JOIN `shop_payment_method` on `shop_payment_handler`.`id` = `shop_payment_method`.`shop_payment_handler_id`
              INNER JOIN `shop_order` on `shop_payment_method`.`id` = `shop_order`.`shop_payment_method_id`
                   WHERE `shop_order`.`id` = '".MySqlLegacySupport::getInstance()->real_escape_string($oOrder->id)."'
        ";
        if ($aGroup = MySqlLegacySupport::getInstance()->fetch_assoc(MySqlLegacySupport::getInstance()->query($query))) {
            $sIdentifier = trim($aGroup['ipn_group_identifier']);
            if ('' === $sIdentifier) {
                $sIdentifier = trim($aGroup['system_name']);
            }
        }

        return $sIdentifier;
    }