There are stores trialling exactly your preferred method. One of my local supermarket chains has portable barcode scanners on a wall. You pick one up, scan your groceries as you collect them, then take the scanner to a self checkout that links to the scanner. At that point you pay for your items and leave.
Best Buy started doing this with their app. I’ve used it multiple times already. It’s so convenient. Scan the barcode with your camera in the app, it adds to the cart, pay when you’re done.
Anecdotal experience: Unfortunately, products that are locked up create a problem. I went in for two items. One of which was a single RAM stick for laptops. The employee refused to give me it even though I was literally going to pay for it on the spot as I had already collected the other item I wanted. He insisted it goes to the register per policy. I quickly got the barcode as he held it, then paid. “There. Paid for. See” as I showed him the screen. Dude was so annoyed as he handed me the RAM.
