Keep your eyes on the register at your supermarket.
Empty your cart of groceries on to the belt at the register before the assistant starts scanning your items. You can ask the assistant to wait till you are ready. The reason for this is so that you are able to keep an eye on the price of the items being scanned.
Many stores have a “Scan guarantee policy” which means you get the item for free or at a discount if the price the register displays is higher than the real price.
It also enables you to check the prices as the assistant scans them. I have found that often an item that has been reduced does not register at the lower price.
This process can save you quite a few dollars over time.