Hi,
The "product custom field" is for extending the product fields. It is the information displayed in the product page.
What you want to do is a "item custom field". An item is a product in the cart, the interaction is with the user in the front-end while the "product custom field" have an interaction (I mean, editable) in the backend.
Item and Order custom fields are include in HikaShop Business.
Product custom fields (and also: user, category...) is a feature include from HikaShop Essential.
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.