private static function _addMetadataToIpnMessage (
        Message $ipnMsg,
        OffAmazonPaymentsNotifications_NotificationMetadata $messageMetadata = null
    ) {
        $ipnMetadata
            = new OffAmazonPaymentsNotifications_Model_IPNNotificationMetadata(
                $ipnMsg,
                $messageMetadata
            );
        $ipnMsg->setNotificationMetadata($ipnMetadata);
    }