Why so long?
Using current tech, there is no reason you couldn't require people to scan their payment method (phone, card, etc...), weigh the shopper and then require they weigh the same when leaving and scan the items necessary to make up the difference. Alternatively, RF tag everything and just require payment for the items scanned.
It's really not hard to do with modern tech. Maybe the cost per RFID tag would be less than the cost of a human to process the checkout.