Each easyPoints plan has a limit to it's active customer count. When exceeding the limit, easyPoints will create extra charges based on the exceeded amount.
The monthly bill may grow unexpectedly if the active customer limit is greatly exceeded at the end of your app subscription cycle.
Please make sure to check your active customer count periodically and switch your plan accordingly.
The currently active customer count can be checked in the easyPoints admin dashboard.
Check this page for the limits of each plan.