Misleading advertising and price mismatch on Item on Sale

Last week I bought a 20L paint which was on discount in store display for R749, when I went to till they charged me price without discount R849 , I noticed and queried and they said it's system error they will correct the price. Today again I bought the same paint and similar thing happened where they charged me full price instead of discounted price which was on display. I bought few things and noticed this only when I reach home. As a customer I trust the merchant on giving product on the price which is on display in the store. When 1st time I reported this I thought it was a mistake and they will correct it, but second time same thing happened, I feel cheated here as how many times customer check price for each and every item after paying. Now I will have to go again to store to get my money back. But as a customer It's hard to trust a store which doesn't fix it's problem when it's reported first and carry on with ********* business practise.