Overcharging/Robbing people

I was over charged by Pick n Pay Newton Park, Port Elizabeth on an item I bought that was priced R59. 99 but on the till slip i was actually charged R79. 99,this I must state was the normal price it was not a marked down price where they could say that there was a mistake on the pricing. I have emailed Pick n Pay head office with the complaint with no luck, they only responded that they will investigate up until today I have not heard nothing from them and every time I email them wanting to know progress/feedback they tell me the same story that they will get back to me or are still investigating. I mean this has been going on for over a month now surely picking up a phone and calling their store in Newton Park shouldn't take this long. They are just busy playing and not taking me seriously.