Hi,
How do you determine what is the product that the receiver can buy for free ?
If it is something "static" (related to the pack), you can use a static link and create one attach serial configuration per pack.
If it is something dynamic, you can use a product custom field in order to store the URL.
By using a product custom field only visible in the backend ; you have the total control on the link and you can set a specific link for each product.
Otherwise ; we will need more details in order to understand fully your request.
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.