Water usage is probably my biggest. Living in a high desert, my wife and MIL see no problem with filling one side of the sink with hot soapy water to wash a few dishes because “that’s just how I’ve always done it”, to watering the grass and plants for hours. All of this makes me mental.
By saying “Thank you” and “You’re welcome” you’re expressing mutual willingness to end the transaction in a civilized way. It’s just a standard politeness, a phrase. No need to read into the original meanings. Nowadays and in the context of shopping you’re just both saying everything went well and you’re finishing the transaction in peace, there’s no implication of anything - meanings evolve.