[Johnny555]

Charging per request for something that has a cost to provide is reasonable (and it's a tiny charge in this case, $3.50/million or 0.00035 cents per request).

Otherwise there's no incentive for application developers to optimize their usage.

In my last company we charged per unit of work, but it was possible to send a lot of requests to do a tiny amount of work -- some customers hammered us with millions of requests to do each unit of work. It wasn't malicious and it worked with their workflow, but we lost money on those customers due to all of the infrastructure required for the request routing.

The company eventually started charging a nominal fee per request, which resolved the issue.