> they obviously do have some billing check that prevents me from making additional requests after that "final query"

No they don't actually! They try to get close, but it's not guaranteed (for example, make that "final query" to two different regions concurrently).

Now, they could stand up a separate system with a guaranteed fixed cost, but few people want that and the cost would be higher, so it wouldn't make the money back.

You can do it on your end though: run every request sequentially through a service and track your own usage, stopping when reaching your limit.