“I hadn’t selected the most expensive OS instance type. I hadn’t even used it much at all.”
AWS’s own Amplify guide spins up r5.large OpenSearch instances at $134 a month each. The sandbox delete command leaves them running, so one hobby project stacked up several domains and a $1,125 bill for work worth about $50. AWS refunded it after a support ticket and fixed the guide about three months later. The default that bills you is never the default that gets fixed first.