We are working on a SaaS that solves exactly this problem combined with some additional things except you don t have to enter any coupons, its completely managed by the back end.
If you're into that sort of thing take a look at how I handle SaaS billing in my GeeTasksLite iPhone app. Try to cheat your way around, too. :) If you need to get in touch with me, just contact GeeTasks support.