egress fees, inter-DC transfer fees, IP fees, ebs storage fees, transactional fees on things that are transactional. If you don't carefully engineer everything you'll get left with a $15,000 AWS bill. This doesn't cost Amazon anywhere near $15k to provide, so they just "waive" the fees if you promise to be a good little user.
Don't disagree with this, but this is the same in almost all cloud providers. Want to get started with Digital Ocean's app platform? You're paying for Container Engine to store your containers, then App Platform Business just to support scale-outs, then multiply this for the number of services you're deploying and make sure you don't have egress overages etc etc.
