Amazon SES costs
Amazon SES charges are based on the total number of emails being sent out from your account each month. When you sign up for a new Amazon Web Services account, you can use their free plan. On that plan, in the first year, Amazon will not charge you anything for the first 3,000 emails you send in a month. If you exceed that, you'll pay them $0.10 for every additional 1,000 emails.
More information can be found in Amazon's documentation.
You will not incur any other charges when connecting EmailOctopus to Amazon Web Services.