if the kid could successfully modify the scanned value of physical barcodes a) that would be quite the feat and b) that would absolutely be showing up a bunch of big names
This attack has been done trivially for years - you just sticker over the barcode with the barcode of a cheaper item in the store. If you plan to use self-service checkouts for this scam, pick cheaper item with same weight or with a tag that prices cheaply per unit of weight (produce) etc.
It wouldn't be quite the feat at all. Barcodes for pre-priced items sold by weight (cheese, meat, etc.) encode the price in the last four digits. Replacing those would be trivial.