Interestingly Elastic IPs are free when you use them, but cost money if you don't use them. That's because there's a limited pool of public IPv4 addresses, and once you request one of these addresses: noone else can use it. So you better use it, or pay for keeping it from other users. Since our VMs are going to be shut down most of the time: this is one of the costs that will come out of your 100$ credit.

You need lots of practice to get used to these concepts. It's not enough to just read about them.