Hi,
Yes, you have to set the additional every time the cart is loaded.
It means that you have to use the session to store your data and re-set the cart additional tip if there is something in the Joomla user session.
When the cart will be converted to an order at this moment the additional will be fixed in the database. But the additionals are not stored in the database (and so, the session) but HikaShop.
For the cart, it is dynamic fields managed by plugins.
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.