Roland Perry wrote:

At all of Sainsburys, Asda and Morrisons I've regularly placed my own
backpack in the bagging area. Once the supervisor flicks a switch it's
perfectly okay to load up the bag directly without having to verify every
single item. Maybe some other supermarkets use less flexible equipment?

The first question Tesco ask you is "Have you brought your own bag", at
which point you can put it on the bagging area and it re-calibrates the
weighing scales.

I must admit to not having noticed that - I thought the question came up
just before payment and was linked to green incentives. But my local Tesco
often has the sound turned off and I just use the machines on autopilot.

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