Pathetic Pick n Pay overcharged me, didnt refund and treated me badly

This review is a bit delayed but I have been moving house - I still have my slips and pics of the items though. I purchased celery from Pick n Pay Malvern a while back and it was marked at 'Buy 2 for R35'. When cashing up I was charged R21.99 each, which totalled to an overcharge of about R9. I usually dont bother about it but that day I thought its rightfully my money and Pick n Pay would not give me my items if I was R9 short! I went to the customer affairs counter and there was only 1 lady being served there so I stood on the outside of her so as not to enter the store with my full trolley of paid groceries. There is no sign regarding the line anyway. Then I see people lining up on the inside of the store - I didnt worry because I thought 1) the staff member had seen me there and 2) the people saw I was standing there before them! Lo and behold once the lady finished and I walked through the Pick n Pay staff member very rudely told me I was not in the line! I told her I was there 1st and everyone saw that but she wouldnt listen and waved me off! I was livid since my time was being wasted by Pick n Pay's incompetence of not charging what they advertised! I went to the store manager, his name is Grant I think, and he was not helpful or sympathetic at all. He said I must just stand at the back of the line! I left because I had been treated badly when Pick n Pay's incompetence had wasted my time and money. I have since stopped shopping at any Pick n Pay - I know it wont dent their business but their service was pathetic when they were the ones who overcharged me! Plus I am sure if one day I am R9 short they wont just let it go and let me take my groceries!