And it doesn't look like there's any sort of "you only used 20 searches so we'll Free Tier you rather than $10 tier you this month".
This is not true, you don’t pay if you don’t use it. I think you’re assuming “credit” means some sort of redeemable voucher, but it’s account balance that automatically covers your next bill. Then at the end of the second unused month you get a credit that covers the third month, and so on.
The thing I’m mainly on about is the UX of not having to babysit another subscription. I once had a newsgroups sub that was exactly like this and it was amazing. You’d say “I want to spend at most X” and it would just ramp up towards X tier as needed.