Price at the till more then shown on the shelf

Don't remember when I was charged at the till the price displayed on the shelves. It most of the time higher than the shelf price. Now they added more "management has asked us" at the till. Although not a pensioners day they asked if I have builders card. Because I said yes there is extra slip to sign, yet the price went from R 85 on the shelve to R 90 at the till. They checked and found I was right. I don't trust Builders Warehouse.