It's an interesting service. I wish you all the best with it.
I fear that I wouldn't use it sufficiently to warrant paying for it. I write more HN and Reddit posts in a week than I've sent emails in the last year.
That said, I'd be tempted to use it as a good proof-reading service for important emails if it weren't too expensive to use ad-hoc.
However there are at least two people I would hope would sign up. Their emails (and my sanity) could do with it!
Likewise if you did Facebook updates too, I'd be tempted to pay for some friends on my feed to use your service.
I trust you'd have no issues with using British English for spelling, if requested?