As far as I've seen, for APIs that don't "consume" any resources, the usual schema is to charge a fixed fee.

Either that or it's just included part of a higher pricing tier. Of course, there can be a quota restricting how many API calls are allowed per period.

Charging per call feels weird when the cost of a call is essentially zero. You only really see that in things that are computationally expensive like LLMs, image optimization, etc.