> 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.

They do have a billing check, but that check is looking at "eventually consistent" billing data which could have arbitrary delays or be checked out-of-order compared to how it occurred IRL. This is a strategy that's typically fine when the margin of over-billing is small, maybe 1% or less. I take it from your description that the actual over-billing is more like dozens of dollars, potentially more than single-digit percentages on top of the subscription price. Here's hoping they tighten up metering <> billing.