Hi,
As I said, it is not a feature which can be provided by HikaSerial but by HikaShop.
What I can recommend you is to use the HikaShop "User points" feature.
Thanks to it, the user will have some points which can be use to buy a specific product.
By buying a $50 product, you will gain 1 point and you will gain 3 points by buying the $150 product.
Using 3 points, you can have a $50 discount on the products (you can limit it to a specific category).
So the user will have his "free" serial but he will have to reclaim it by using his points.
Hope my explanations are clear enough.
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.