The parent commenter's concern was that the pricing is difficult to understand. But you said one of the reasons you selected your pricing structure is so that users now exactly what they will be charged for. I agree that it's much easier to estimate sales then the number of API calls that will be used.
Another point to consider is that users have a monetary incentive not to use any new features you may come up with in the future. (Unless of course, they have strong reason to believe that implementing these features will provide a significant boost in sales.)
