https://smartshop.sainsburys.co.uk/
Is really easy. They occasionally check 10 items of your shopping, especially the first shop, but I've only been asked twice
I use the equivalent system in Waitrose and it's brilliant. So quick and easy and you don't have to move shopping umpteen times. Just pay for it at the machines and go!
I've used Waitrose Quick Check for years...on the odd occasion that the system requires a re-scan, they now just pick a few items at random and it takes seconds.
We use it whenever we shop in person and really like it. Since coronavirus appeared we've largely used deliveries but we prefer to shop in person and will be back to Smart Shop when the virus permits.