That's pretty much what I came into this thread to say. The thing I'd add is, DynamoDB is pretty nice if you understand how it's meant to be used - a relatively dumb key-value store with good persistence and table-size scaling to the sky. Definitely don't attempt to use it as a SQL database.

The best way I can come up with to rack up a $75 bill for some prototype code is to vibe-code a thing that attempts to treat it like a SQL database with JOINs and GROUP BYs etc. Or similarly write code against it absent-mindedly with about as much understanding as a 2-year-old free AI tool.

Where it really shines is use-cases like I need like 1 or 2 simple relatively small tables of persistent storage and don't want to deal with a full RDBMS. Or I need 1 ridiculously huge table to be queried in a relatively simple way, and don't want to deal with fitting that data into a RDBMS.