How do I give a customer credit for his account for an online business?

[RoyalsOvaCards]

I want to open a business that gives the customer credit to spend as he or she likes, sort of like ITunes. I want them to be able to log into their account and be able to add five dollars to their account to spend sort of like buying apps or songs. I want to know either how you design that or what programs allow you to do that to your web page. Advice helps too.