Well, if this is provable and significant, here's an idea for a YC startup -- handwritten than you notes as as service (HTYNAAS)! Perhaps a pen plotter can write them, and some AI net can give it the human touch.
If this piece of wisdom becomes well-known and every shop does it, then it won't be special anymore and customers will ignore those pieces of fluff.
Then again maybe shop owners will feel rude if they don't join the "common courtesy" of putting in thank you notes, and so your HTYNAAS might have a chance after all. You just need to guilt-trip them into buying it. I say this tongue-in-cheek, it'd be awful if we wasted money, time and paper because "thank you for shopping" notes became obligatory.
Then again maybe shop owners will feel rude if they don't join the "common courtesy" of putting in thank you notes, and so your HTYNAAS might have a chance after all. You just need to guilt-trip them into buying it. I say this tongue-in-cheek, it'd be awful if we wasted money, time and paper because "thank you for shopping" notes became obligatory.