Looks cool though.
I have also designed my own billing system, which has server-backed "free trial", time-limited purchases, and of course "we want to thank you this much" free orders. You can see it in action in my GeeTasksLite iPhone app. I'm thinking about selling this billing system as a library and a service (the server side).
Does it invalidate a coupon after it's used?
What would happen if I get a new phone? Would I need to get a new coupon?
This seems like a good solution until they implement it into the market itself.