Shit, I'm in the same boat with energy drinks. All the cashiers make jokes about how they already know the price. It's like a roll reversal of customers making jokes like "that means it's free, right?"
I hate it, but I know their job sucks so I don't say anything.
I used to have the same thing with a pizza place near an old house I lived in. One night, I ordered a pizza. Got a call shortly later saying that they'd give me a pizza that was bigger than the one I ordered for the same price if I went over and picked it up myself. From that day I didn't order near as much pizza from them.
As someone with major social anxiety, I hate when this happens. I can continue going there and endure agonizing awkward small talk about my order, or never go back again. Its usually the later. Ive lost a lot of good places like this.
Throughout my years, I have dislocated my shoulder fourteen times so far. I was a frequent visitor to the ER and eventually found out they nicknamed me "shoulder guy." (Especially back then, when I was hurt or nervous or on painkillers, I get chatty. I guess I made an impression.)
After the visit where I found that out, it was years before I dislocated my shoulder again and by that point I'd moved and so went to a different hospital. Perhaps I was subconsciously so mortified that I avoided it for that reason.