With the handheld ones (first picture) you just scan the products and put them into your bag when you pick them up at the store aisle. To pay, scan a special barcode at the checkout and put your card into the machine.

No need to rebag your stuff, shows you your current total to pay at all times, and it's simple enough that I've never seen it out of order. Oh, and there's no risk of getting stuck behind that family that apparently wants to get the whole year's food shopping out of the way.