1. An API/plug-in: no. A service where you do most of the work: yes. I.e., I upload a spreadsheet and you make the magic happen. I don't want to have to do any extra work than I have to, especially since I'm paying you money. Still, this would be very valuable (akin to Shoeboxed entering all my receipts digitally for me).
2. There's only one feature that is important to me: there must be no doubt whatsoever that this letter was sent by anyone but me. I'd expand on this but I think you already have a pretty good idea of what this means and how to solve for it.
Great use of the Woothemes template by the way. There are definitely some UX improvements that could be made (see: http://www.slideshare.net/shereefb/recommendations-for-bette... for tips. Original conversation: http://news.ycombinator.com/item?id=1836538). But overall you seem to be on the right track.
As a small business owner, this definitely solves a noticeable pain point. TTN is a service I would use, at the right price point.