[fixie]

Be sure to check out Waypoint (usewaypoint.com). It's an email API with a tightly integrated template builder and has idempotent requests (https://www.usewaypoint.com/docs/idempotent-requests).

[bgentry]

Thank you! I'm not sure how this didn't come up in what I felt was a pretty comprehensive round of Google searching, but it looks like a fantastic option. If only they had free allotment or something <$20/mo for a small number of emails :) But otherwise this looks a the solution I was looking for.