I didn't take it as rude! I appreciate your input. And I agree that the billing will seem more complicated until and unless I can win enough trust that users realize the costs don't matter, because they'll come out ahead.

You're right that sending/receiving don't add to overhead much. The receipt is actually way overinflated right now, and should be down to a more reasonable number tomorrow (about $0.03/1000). I might just waive it completely if it complicates things too much.

Sending is much harder, because I need to deter spammers. I priced it significantly above Mailchimp's price for that reason. But the cost will definitely go down as my ability to detect and ban spammers improves, and we'll likely end up pretty much with a scheme like you're proposing.