wow, they are really tracking from where you're calling the API to give credit, first of its kind I've heard :)
It's not that hard or far-fetched, given that your EC2 instance needs to have permissions to call SES and AWS can tell whether these requests come from inside or outside their network.
> 62,000 Outbound Messages per month to any recipient when you call Amazon SES from an Amazon EC2 instance directly or through AWS Elastic Beanstalk.