If someone was exceptionally nice to me, I would scan the barcode as if they had an invitation.
A few supervisors knew I did it and looked the other way. I would have done it all the time if given the option. Rewarding behavior encourages that same behavior in the future.
If you want your customers to be nice to your staff, reward the nice ones and kick the mean ones out, meanwhile offering products that customers can’t resist. Next thing you know, everyone will be nice.