customers come in groups (lunch hour is when many shop) so the free time is a different part of the day and you send them home. When a clerk is counting cash they could serve the next customer. If there isn't a customer they could stock shelves or clean. stores might have one clerk doing nothing at a till but there are several more scattered around the store doing something else while waiting for a help needed call.

You also completely discounted the managers time counting cash in the backroom. That is more than an hours work in my experience, time that could be used for other things.