Builders Warehouse does it again again and again

This is happening too often, at too many Builder Warehouse and Builders Express branches. This time Builders Warehouse Somerset West. I took a 5 litre tin of SupaSeal from the shelf marked at R339.00 at the till it scanned at R380.00. As usual I accompanied the cashier to the shelf, pointed out the bar code, back to the till, the supervisor was called and eventually I paid the price as marked on the shelf. My time wasted and a number of irate customers waiting. This has happened to me 6 times, at 3 different branches. Is this Builders Warehouse's National policy? I wonder??