This holiday season, the hottest trend among retailers isn’t found on a store shelf. It’s taking place at the cash register.

Major retailers, from Best Buy to Toys R Us, are promising to match their competitors’ prices. Generally customers just need to bring in an advertisement or printout to prove that the same item is available elsewhere at a lower price. In some cases, shoppers can come back with a receipt and get a refund for the difference if the price of an item they bought fell.

Best Buy Co. Inc., Target Inc., Wal-Mart Stores Inc. and Sears Inc. offer price matching to customers year round. But what’s different now is that Best Buy and Target are matching online retailers such as Amazon.com for the first time. That’s a big deal, since online prices tend to be lower than those at stores.