Hi,
Did you customized your email ?
As explained in some other thread, the notification email call a trigger that HikaSerial would use to display serials.
<?php
JPluginHelper::importPlugin('hikashop');
$dispatcher =& JDispatcher::getInstance();
$dispatcher->trigger('onAfterOrderProductsListingDisplay', array(&$data->cart, 'email_notification_html'));
?>
HikaShop calls some triggers when it sending an email (onBeforeMailPrepare and onBeforeMailSend). It is possible thanks to a plugin to catch this event and modify the email content (by adding an attachment).
onBeforeMailSend have two arguments, the first one is the hikashop mail object, the second is the Joomla mailer object.
You can use the function
$mailer->AddAttachment($filename);
To attach a file to the email.
The $mail object contains some values like subject, dst_email... And $mail->data.
During order notifications emails, data is the hikashop order object.
Regards,