In the majority of US supermarkets, there are receptacles in the parking lot for carts. You just find the nearest one and push it in there and leave it after loading your car. It saves a trip back to the store. Later, a store employee will push a line of 20+ carts back all together to the store.
We have this in most supermarkets in Europe, but the receptacles have the chain and key for retrieving your coin. Employees typically carry a fake coin on a keyring for bringing the lines back to the shop.
(Supermarkets also sell branded versions of the fake coins for customers too accustomed to arriving with no spare change for a trolley)
