You insert a coin into a slot in the cart, and a chained key releases the cart from the next cart it's chained to. When you want your coin back, you return it t

You insert a coin into a slot in the cart, and a chained key releases the cart from the next cart it's chained to. When you want your coin back, you return it to the line of carts, stack it and then insert the key from the next cart into yours. Your coin is released and the store doesn't have to pay someone to collect them.