Clicks overcharged me again!

Clicks Rivonia Village had me serval times complaining to the store managers about the price I get charged on the cash-out till, which is different to the advertised price tag on the shelf. I feel cheated, that I have to watch this on the till and then bring this to the managers attention. Should this not be done right first time by the staff responsible for the marked shelf display? This happened now several times when I have been to this store, a very bad experience for me, (finally I get charged the price advertised on the display.)