Incorrectly Charged

I came to buy Simba Chips 125g. The special was R10 per packet and it wasn't specified which flavors were on special when I went to pay at the till the cashier said the other bag of chips is not on special and I have to pay original price for it. I asked how come because the ad says otherwise and she said there's nothing she can do about it because the other one has a different expiry date that's why it's cheaper. I had to pay normal price for the item. I'm very disappointed and I would like this to be reimbursed Dis Chem La Lucia Mbali-***