[mayniac]

Supermarkets here usually require an employee to approve backpacks. You put your bag on the scales, hit a button saying it's a bag, then an employee comes round to confirm.

In practice, it usually takes longer to get an employee's attention and have them come press the button. So I don't bother. It's usually quicker to bag everything at the end, and I tend to get fewer "unexpected item in the bagging area" issues when balacing everything on the scales.