1. Do you do international? I see a lot of need for letters to Mexico, China and India.

2. You could charge a lot of money for handwritten notes. In fact, I would pay $20 for someone to hand write a letter to my grandma, once a year, or so.

3. CC from email. It would be great if I could simply cc xxxx@snailpad.com which you would print and physically mail out.

4. It would also be useful if you could print and snail mail some documents I email you (receipts, hotel reservations, maps, etc).

Edit ... oops I see you have a lot of these features already. Maybe you can work on #2. Cheers!