A common practice here in Israel
(and probably other places) to encourage shoppers to return their shopping carts
to the queue:
The carts, while standing in the queue, are stacked together. From the back of each cart, a chain with a metal piece stretches to the front of a special lock on the next cart (that is, the cart behind it). The metal piece, once inserted, is secured inside the lock, locking the queued carts together.
In order to remove a cart from the queue, you have to push out the metal connector by pushing in a coin from its' opposite side. Your coin would then be locked in the cart for the duration of your shopping. Upon returning the cart to the queue and pushing the metal piece (belonging to the last cart in the stack) back, your coin would pop out.
The coin used in Israel is a 5 sheqel coin, which to the date of writing, amounts to about 1 US dollar. Not a big loss, but who wants to be the "sucker"?