You ad to sell

My Project is very simple maybe it does not worth asking for support but i'm just a creator and novice in scripting matter... My need concerns a specific app who offer to users the choice between 4 or 5 options to buy a set of credits (points or stamps) so that it can be used several times. And each time a user buys an item, he should know how much credit he still has in his account, as well as the data in the server.