Here's an idea I had to keep the hand written allure and still eliminate the major hassles (stamps, stationary, envelopes, etc.).
What if a customer signed up and was given a template to print out on any old printer. This template has a To/From section at the top similar to a fax cover sheet and below that, a large rectangular outline.
The user would fill out the top contact info, hand write the thank you note inside the box, snap a picture with their smart phone and then email the picture to you. Your servers would identify the user based on the email address, place the hand written contents of the rectangle into a gorgeous thank you note template, OCR the contact field at the top of the page and then email a sample copy to the user for approval. If everything checks out, user clicks 'approved' and copy gets sent to intended recipient.
Obviously that is an over simplified explanation and a good amount of work would need to go into something like that, but I can absolutely imagine having a stack of these pre-printed forms sitting on my desk and scratching out a quick thank you note to people.
4 for $1, 50 for $10, unlimited for $100/year? Or do freemium where you charge for premium or custom templates, remove thankuz branding from the email, etc.
You'd probably also want to do some image processing so the hand written section looks like it belongs in the template and wasn't just pasted in.
Hopefully that brainstorm is useful to someone, good luck with your business!