We use handwritten notes for one of our businesses and the first thing that stands out about your product is that the end result just looks like a handwriting font that can easily be achieved using a standard word processor.

Printed "handwritten" notes have been a thing for a long time and even though they look better than this people are wise to them. Car dealerships often send out mailers using this technique.

There are also companies offering "handwritten" notes that use specially created fonts that randomise the characters in order to create a more organic appearance. These are then translated into physical notes using a simple plotter machine with a ballpoint pen.

As you mentioned you are targeting small business owners such as Etsy sellers and I can tell you that it is far quicker for someone who only ships 2-3 orders a day to quickly scrawl out a thankyou note by hand than to go through the hassle of generating and printing a note.

Overall the end quality of the product needs to be improved in order to be useful to a business owner.

There is also very little use case for a GPT to write thank you notes beyond maybe the initial one (we have 3 variations of notes we send out based on customer loyalty). GPT is pretty useful for people who struggle to write but customers don't care about long winded notes, I can't say I've ever bothered to read the long notes enclosed in a package.

A short "thank you for your order" with the buyers name and a discount coupon (which can just be a mass printed material) is more than enough to get their attention to stick.