Hello,
Why not setting a free price for these variants ?
In the case where your main product have prices ; you can still see to set a free price on the variant by adding entries.
By having prices in the variant configuration it will not load the price of the main product. And if you really need to add a "0" price ; you can see to enter the value "0.00001". The rounding will mark the product as "0.00" so it will be considered as "free" even if it won't be a real "free product".
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.