Using in game store Credits?

So i have it set up where our database stores the amount of credits that a user may have purchased from us.

what i am needing help with is:

1) I need the user to be able use the credits to make future purchases

2) when they make a purchase i need them to be able to enter an item name and the amount they want to spend towards that

3) i need it to automatically update the credits the user has

4) and i update a database that will list all the items requested. we need this database to work...

Using in game store Credits?