You can use the API. You're using the fixed price becasue it's cheaper. It's cheaper because they price it assuming a certain amount of usage being below a threshold. The usage went above the threshold. The usage was limited because of that, and they will make a plan with an increased fixed price and/or increase the price of the pro plan.
Agree with the other reply here. You are complaining about a problem that does not exist. Want price predictability? Use the API pricing! It is pay per use.
The Buffet-style pricing gets you more bang for the buck. How much more? That bit is uncertain. Adjust your expectations accordingly.
if you feel cheated now, I promise you'll feel more cheated if they did this after you rely on it for several months. the least worst option is to change pricing as early as possible.
can someone please find a conservative, sustainable business model and stick with it for a few months please instead of this mvp moving target bs