We use http://sendy.co/ - they have a one time fee per domain and needs to be installed on our server. The best part about sendy.co is the beautiful reports. You should check it out and try to integrate some/all of them in markdownmail.io to make it more effective.
All the best.
Part of the problem we had with sendy was just general stability with the amount we where sending. All of sudden you need to support a mysql and php db. It was just another thing to worry about. So we thought if we have to worry about it lets write it ourselves.
But the graphs are easy to digest, and the generated CSVs (list of people opening the mail etc) are very helpful (at least to me).
Any thoughts on pricing?
Also, I'd reach out to AWS guys and ask them to provide $50 (or $20 - whatever they agree to) credit to every customer that signs up for your free tier since that'll lead the guys sitting on the fence to explore your product.