Things i think about:
In the comics, every once in a while, Superman will purchase something at super speed. Like fly through, grab what he needs and leave the cost of it on the counter. I like to think that in with the money is a little slip of paper with an S shield on it so the clerk know what the hell just happened, and this is common knowledge. So all over the world shops and stores proudly hang these little slips of paper in frames in a prominent place in the store/restaurant.