Being charged different price from advert

This afternoon I went to the food *****s market in boardwalk Richard's bay. I had seen the sign buy any 3 punnets of g****s for R50.00 they did not have all the advertised ones. However an employee at the store said that I can even take 3 of the same. When I went to the cashier I was being charged for individual punnets. When I queried this and said that I don't want the items and to cancel the sale. I had also requested to see the manager. He said that they have been selling the product all day and there was no problem. This is an on going issue at this store. How many customers haven't noticed this.