So once everybody has automated this, the next level will be the handwritten note with a personal detail, which will increase (repeat) sales.

Back in the day, I used to write a handwritten note with every single box I sent out. Since I had often had a conversation with the customer prior to that, I could often include some personal detail, showing that the message was not generic but truly written for that person. (I did have a generic message that I would use in a minority of cases.)

It does take a little bit of dedication and love of the customer to do this over and over again, so I'm not convinced shady, unscrupulous marketers would do it.

Caveat: My business eventually failed, so...

That depends on if they are handwritten, or some font that looks sort of like handwritten. At least until/unless AI can write thank yous that pass the Turing test. Right now the effort vs reward of a thank you is not worth it for most sales, and people can pick out computer generated thank yous.

Even then, Wal*Mart will never send you a thank you note - the cost of postage alone is too high. However car salesmen have long ago figured out it is worth writing a thank you - it only takes a few repeat customers to make the time and postage worth it.

That said, if you are a small business a handwritten thank you is good advice. Very few small business run a many customers with a low profit business model (only retail), and as such you are probably in one area where a handwritten thank you will help.