Oh, you do, tigermoth,it's wonderful - works v well with things like sausages on BOGOF, say they're £1.99 each and you get 2 for £1.99 so it knocks off 1.99 for each 2; if they get reduced to 99p each you make 1p on the deal, and if they get reduced beyond that it's all profit!
When our store first opened the stocking levels (ie levels of stock, not tights!) were completely mad and the manager kept reducing things by about 75% to get rid and then the computer would knock lots more off, I don't think we ever ended up giving people money at the end of the transaction instead of them paying but it came close a few times!