protected function calculateChecksum($orderNumber, $timestamp, $paid = null, $method = null)
    {
        $data = array($orderNumber, $timestamp);
        if ($paid !== null) {
            $data[] = $paid;
        }
        if ($method !== null) {
            $data[] = $method;
        }
        $data[] = $this->_apiSecret;

        return strtoupper(md5(implode('|', $data)));
    }