Wrong contract supp****

In October 2014 I upgraded to the apple iPhone 6. I wanted to upgrade to the mychoice 100 @ R499 but instead the consultant at the is I store upgraded me to the wrong contract. Which ended up in me paying R1200 instead of R499. I went back to the I store to speak to the consultant Tracy Lottering but she is very stubborn and won't admit she made a mistake. Then I went to MTN to see if they can fix it seeing as they are the service provider. I lodged the complaint with a lady called Shamunah. She lodged two complaints one for my contract to be fixed and two I never received my megabytes with my airtime. After a month I phoned the call centre they said Shamunah lodged the complaint incorrect. And best of all they cannot trace my contract. I have been struggling for almost 3 months now and no feedback. I tried to get the contract myself at the I store but they said they sent it to MTN already.