I Followed the Official AWS Amplify Guide and Was Charged $1,100

Every official AWS guide is designed to make you use as many AWS services as possible, which increases the risk of spend. You have to be extremely critical of anything they recommend (GUI defaults, CLI tools, guides, recommended architectures etc).

There's a reason there are very well paid positions in companies to guide colleagues on how to use AWS cost-effectively and with lower risk.

Exactly. And for small scale deployments or tests, the most expensive parts are almost always the ancillary things or the newfangled services they recommend in lieu of something simpler.