The one I hear, invariably, is "have a good one." So irritating. Have a good what, exactly?
I think the practice of clerks saying please/thank you at the retail level have fallen out of favor because it implies servility, and no one wants to be made to feel servile. I would bet they also reason they don't even have to say "thank you" because, after all, you're giving money to the store, not them. They just work there to transfer it from your hand to the till. They do not see themselves as representatives of the store, even if you in all fairness regard them as such.
One habit I have refused to fall into is to thank them when they give me a receipt, because that's when I hear "no problem", as though they did me some favor. I hear customers thanking clerks all the time. I can only hmph, smile, and shake my head when I see it. Back-asswards, as they used to day.