I also am running a Tor relay on EC2, and I was under the assumption that I wouldn't have to worry about charges with a stock install. Can you provide more detail about what I should do to my instance to prevent charges?
Edit your your /etc/tor/torrc
And change the AccountingMax line to:
AccountingMax 3000 MB
This will reduce the monthly usage to 12GB and keep you under the 15GB limit. I've been running this bridge for a few months now and have never gone over.