Hello,
You can try to resent the confirmation email by changing its status two times (and check the box to notify the customer the second time).
You can also add a product download file to let the customer access to the voucher anytime.
But yes, when you send an email via the HikaShop backend regarding the order details, the plugin does not include the attachment for that email.
It's a filter within the plugin and it would be difficult to "control" it when you redact the message.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.