Hi,
HikaSerial includes a plugin called "attach serial" which work with the "serial" feature of HikaSerial. It allow to include an image in your email with some data including the serial generated by the system.
In your case, you want to display a custom message in the email, does this message would be always the same for all gift certificates ?
If so, an override of your email would be easier.
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.