I don’t get this. You guys had two years to detect the disparity. It is rather obvious that nobody on your end bothered to setup infrastructure essential metrics
Firebase is not making up any new charges; they simply did not catch the glitch.
If you have been using a pump that serves two gallons for a price of one for two years, you probably should not demand a refund once the pump is fixed.
This is a great $2500 lesson.