Hello,
You can enter a subject for the expiration notice email.
If the subject is empty, it will be automatically use the translation "SUBSCRIPTION_EXPIRATION_NOTICE_SUBJECT" as the subject of the email (you can see how the override is made in the preload part of the email).
We could change the behavior and use the Joomla translation system on the expiration notice ; but since we did not have enough feedback on the possible parameters that we could use ; we prefer to let the subject empty and give the flexibility to override via the subject and the preload (for something more complex).
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.