Once, skiing in Vail, I needed glove liners. The Evergreen Lodge, nextdoor, had a small gift shop in the lowest level which had stuff like that, so I ran over there. When I stepped in, the desk clerk had a sad expression on his face and the cash register door was open. When I told him what I wanted, he said "The computer is down." I asked him if we couldn't just do a cash transaction, that I didn't need a receipt. He said "Then I wouldn't know how much to give you back." I told him the amount, off the top of my head, after adding tax. He brightened up and said "That'll work!" I gave him the money, he made change and I was on my way...