Pretty cool - the only people I send letters to are my family so I'd be a little afraid of them recognizing that the letter wasn't written in my (chicken scratch) handwriting. Do you write the letters yourself? I'd recommend adding a screenshot of some sample letters so we can see what the writing looks like.
I'd also recommend letting someone select the letter, enter the message, delivery date, recipient address, etc. before asking for their billing info. It takes too many clicks right now to actually see how the process works.
Good luck!
