Most kids who work behind a cash register today don't have to really count...the register tells them what bills and coins to give back.
I went nuts at McDonald's in TX. I'd go in and order a double cheeseburger to go. "Do you want that on the meal? Is that for here, or to go? Anything to drink?"
One day, I just said, "Pay attention, because I'm only going to say this once: I. Want. A. Double. Cheeseburger. To. Go."
They are taught to ask things like "Would you like fries with that?" or ask about a drink. It boosts sales.
What gets me mad is if you don't want the drink, sometimes they will charge you ala carte instead of meal deal.