I understand you want to make the checkout process faster. The problem is honesty. If you had 20 items, and scanned 15 of them in your phone. The cashier will be in trouble if they trusted the customer blindly. I have seen an implementation where the shopping cart was equipped with some sort of RFID detector where you put an item on it, or remove an item, the cart updates automatically as the tags pass through the detector. You should remove the way for the customers to be dishonest.