At least here in the UK a lot of larger supermarkets give you the option of taking a little handheld scanner with you and doing it as you shop. When you go to the till you just scan a barcode that's on the till and it connects the till to the scanner so you can pay for everything. I don't personally use it because I'm too disorganised a person to pack as I go and also remember to scan everything, but it's fairly popular. It typically exists as an alternative way of using the self checkouts, the option to scan everything at the checkout itself is still there