Honestly speaking, we as shopkeepers have to be honest. Sometimes customers could be careless and give us more than is needed.
We would always return to the customers. Never once we keep it. But do customers do the same? Some do and some don't.
When we give extra change to the customers, some return to us.
Recently, a customer just bought two items but my staff packed for him 3 items. And he never returned to give us back the extra item..