The quarter is NOT redeemed at checkout. You take your empty cart back to the cart area, lock in back into place and Voila! your quarter pops back out of its slot. One in a while, when it is busy, the clerk will swap you cart with its quarter in place for another cart with its quarter in place. You return the cart, etc.
The airport in Vegas had that 25 cent deposit refund, for the luggage carts. One time I flew in, knowing I would have to wait a couple hours, for my buddy to get off from work, to pick me up. I was doing $o well, returning carts, I was kind of disappointed when he finally arrived.