Don't feel bad, I'd forgotten this one: "Using a public or Elastic IPv4 address => $0.01 per GB". I'm sad we still don't have a proper split horizon DNS to encourage internal IP usage, but I can't imagine charging for it in the same zone.
All these cloud offerings are priced so complex you can pretty much expect some nasty gotcha unless you spend an inordinate amount of time modeling your workload and traffic patterns.
And if something changes you can do it all over again.
https://aws.amazon.com/ec2/pricing/on-demand/