With a suitable smartphone app you could dynamically generate the appropriate barcode on screen, with a set discount (say, 50%). Then just hold your phone over the actual barcode as you scan each item.
This should be relatively hard to spot for any cashier watching, and the weights and stock etc. would all match up.
Of course the CCTV cameras are likely to see you and they're likely to spot what's going on soon enough to cross reference before the footage is wiped.