doing battle for shelf discount at till

another one of many bad customer experiences, tried to phone to talk to manager - no answer at ***. It is a regular occurrence that I have to argue about the shelf price versus the till price. Like just now again. Parmalat Easy Gest milk comes with a 10% Smart Shopper discount. I had to do battle with the lady supervisor as she refused to allow the 10% discount. She said it was only on long life milk - duh? The 10% discount tag I fetched spelt it out that the offer was correct. I had to insist on talking to a manager (once again), and she came back after a while (the other customers had to find another till in the meantime and, the customer closest to me said that they experience the same thing with Pick n Pay). Long battle - she came back yes the manager said you can have the milk at the advertised discount. Okay the thing is by now I was so gatvol that I just left my shopping and turned around and left (this time). What a waste of time and energy! Why the hell is this almost the status quo in William Moffat Pick n Pay Hypermarket?