I got a form thank you note from a clothing store in the mail yesterday. It’s pretty clear every customer gets one.
I think that’s a little like writing a thank you note on the back of a check when you get it for your bar mitzvah.
For most people, a thank you is a thank you when it’s real, personal and honest. Hard to do when you sell stuff all day long to total strangers, though. I’m not sure that a non-thank you is really worth the effort. Sure, be kind. Be grateful. But don’t send a facsimile of a personal note when that’s not what it is.