Just address your envelope, write your letter, verify
its appearance, then pay using your Paypal account.
You have a bunch of '2' showing up all over the place.
Mention that you don't retain the contents of the letter once printed (if you don't, and I hope you don't).
Include a photo of a sample letter so people can see if the paper really looks greenish (joke). But really, people want to see if the printout will look professional. And a photo of the envelope too - with stamp.
Mention where you are physically located, so people have an idea of how long delivery will take.
What happens if the letter is so long it takes many sheets of paper (i.e. more stamps) - is there an additional charge? Can I download a book, paste it in and you'll print and mail it to me? Just mention "up to 5 sheets of paper" or something like that, in the pricing page, but don't bother programming it, handle it manually.