Hi,
Yes for the moment the best to select several products is to use a category (which can be unpublished if you want an hidden category).
The improvement of the discount in order to support several products (like several categories) is in our TODO list.
It will come thanks to new options that we will introduce in HikaShop after the 2.3.4 : the namebox system.
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.