MTN Black Friday nightmare

My Black Friday deal resulted in an overcharge of R2218 for December. After numerous visits to MTN shop, the charges for January onwards were corrected to R309 p.m., but I have been unable to receive a refund of the December overcharge which was debited to my bank ac. The cost of travel and time to the MTN shops on so many occasions now probably is more than the money I am trying to recover.