Bad customer service from a cashier /supervisor and assistant manager

I went to pick n pay and purchased Simba Chips that were advertised on the shelf for R27.00 for 3,when I got to the till I was told the special finished on the 24th and today it's the 26th they were still advertised at 14:40. A cashier/supervisor Thandi was called and she told me that they can't change it and she had a very bad attitude and went on to call the Assistant manager Tshepo who also said the same thing again. I fail to understand that the special that finished 2 days ago was still on the shelf at that time of the day.What happened to pay for the price you see on the shelf, I have never experienced such a bad customer service and attitude like I did today. I had to leave everything behind I wanted to buy because of how I was treated at pick n pay Cresta.