False advertising

I bought a hardboard door from Builders Warehouse Bedworthpark on the 1st of June. The board with building material prices at the building department showed the price to be R189. When I got the till I had to pay R275,40. I queried this at the building department but they refused to give it to me at the advertised price and said somebody forgot to change the price on the board as the special ended earlier that week.

How can they indicate one price and charge another price?