Voucher not added and charged full price

Near Christmas, my mom and I went shopping at the Cresta store. My mom had a voucher for R500 and decided to buy towels and a bath mat. When we went to pay, it took forever. The trainee cashier didn't seem to know how to deduct the voucher amount from what we had to pay. He even asked for help. It was taking so long, I went walking around the shop to pass the time. Finally it was sorted and we left the store. My mother paid the balance on her account. When she checked her account balance, she saw she was charged the full amount. She phoned the call centre and was told she had to go back to that store and find the voucher. Have you ever heard something so unhelpful? How on Earth will anyone find the gift card that was probably discarded that day. Why should employee incompetence cost my mother R500? Obviously I had to take drastic measures by writing on Hello Peter to get results as the call centre is useless. Don't make your mistake our problem!