Bad Service

I went to pick 'n pay at Maponya Mall and when I got to the bakery section I saw they had nice doughnuts and they had buy one get one free sign next to them and I tased the lady that work at the bakery for assistance and she was very kind and assisted but when she gave us it the total price was R50 cause i took 6, so I asked about the buy one and get one free she said they will fix it at the till of which was never done and later when I got home i realise that i was charged the full price, Im really NOT happy