Depends on who your customers are. If their customers are strictly other businesses, you should set a base rate limit which is already included in the base fee

Depends on who your customers are. If their customers are strictly other businesses, you should set a base rate limit which is already included in the base fee and only pass the technical costs when the limit is exceeded. In other cases, charging per API call won't work because no business owner likes such unpredictable cost centers anyway (i wouldn't permit my engineers using such a service).