Buy X of Item A to receive X of Item B
 

I'm wondering if its possible to add a script to certain shop items so if people buy a certain number of item A, they get 1 (or however many you want) of item B.

This idea comes from Pokemon where in most games, you buy 10 PokeBalls and receive a single PremierBall.

I'm aware its possible to receive a reward for a certain number of clicks with a certain mod which made me wonder if the above idea is possible.