Incorrect Pricing - Error or Deliberate

I have had this issue quite a few times with Builders and have had it again twice in the last few days. I raised a complaint with Builders via their web site after the first incident a couple of days ago - NOTHING heard, and now it has happened AGAIN!!! Don't know if anyone else experiences this, but with Builders Warehouse Somerset West it happens that I buy something with a certain price marked on the shelf, and when I get to the till, that item is rung up at a greater price!!! First it was small bottles of paint tint at R30 where the till rung it up at R37, then today I bought some larger bottles at R45 and these were rung up at R52!!! Fortunately I queried this, and the price was adjusted - however my Wednesday pensioners discount was NOT taken into account. My question to Builders is .... 1: Do you do this deliberately to make a few extra bucks profit? or 2: is your computer system wrong? or 3: are the people who put prices on the shelves not up to date? It seems to me, as this happens quite a lot, that option 1 may be a possibility!! Either way this is not a happy shopping experience!!!!