Uhmm.. you will have to keep the record somehow manually if you want to work with Foodomaa as is (with no modifications).
I like the idea, i think it's pretty nice, but if i had to implement it, maybe i would use wallet; but once again, i dont know if that would solve your problem because you might have several stores, some of them probably not taking part of this subscription method (im guessing here).
If client pays $100, simply add $100 to his wallet, and then he will be able to make purchases with that money.
Other way of doing it, that i can think of, is using coupons; with those you can chose the amount of uses they can make of it; as well as expiracy and some other cool stuff that might serve you in this case.
I dont know, it is just what i can figure out; there are probably better solutions out there, but i think they will all involve programming...