Contract incorrectly billed

I have been a mtn customer for the last seventeen plus years, it is a business account to which there are three contracts linked. The relevant contract is now in its third year, this contract was for two years on a fixed amount. for R199.90 per month. During January I upgraded my contract to a package of R278,98per month, for one of my workers, Mtn CHOICE 100. As I am the owner of the business and my bookkeeper are in charge of accounts I was not aware of the difference in contract amounts on the account, untill my bookkeeper recently informed of bill for R4 282.02. On investigation I was informed that this upgraded contract was opened and not blocked, as a result therefor after the actual minutes, data and sms bundles were used there was no limit applicable. Three weeks have passed and I am still waiting for an answer from MTN Checkers sentrum, Welkom. Can someone please assist me to solve this problem.