Going back on double the difference promise

Albertina, Dikeledi, and Glenda at Pick n pay Menlyn refused to give the "double the difference" refund on a 3 for 2 promotion on sliced ham that did not go through as 3 for 2. I thought Pick n pay offered the 3 for 2 refund to protect customers against these kind of mistakes. In addition Albertina's attitude was extremely unhelpful.