You could say, "I never thought that it
was a problem. I was simply thanking you". A few of those, and even the most dense of teenagers might get the message.
My personal breaking point comes when some young idiot behind a counter calls me "boss", as in, "no problem, boss".
I usually tell them, "I'm not your boss. But if I
was your boss, I'd tell you to stop addressing customers as 'boss'. 'Sir', 'Ma'am', or even nothing at all would be preferable to 'boss', 'employee'. For chrisakes".
