So we are looking at 14 + 4 +1 = $19 before you even send/receive emails.
I just did a quick look and it looks like I receive around 6000-7000 email a year. Most of it is advertising and notifications (that doesn't count SPAM emails which are countless and I hope you don't charge for). That's an extra 1.4 bucks.
I send around a thousand emails a year. That might seem much but it is actually very low. It is only 3 emails per day and you can do much more if you use email for personal and work. That's an extra 4 bucks.
So total is $24.4 assuming you stop your billing there. That's half of FastMail offering for a beta product which from the looks of it offer not interface.
I might come off as rude but I think you need to remove the pay-as-you-go billing and just bill something reasonable for a whole year. Plus offer something more than "Just Email"; like have a differentiating feature like security or privacy.