Incorrect billing

I've had a contract with mtn since 2019 August for R150 p.m. On the 18th of May 2021 I got a call from Rewards offering an upgrade which I accepted and was going to now pay R200. On the 24th of May I decided to cancel the upgrade, I was not expecting any penalties since it was within the cooling off period. To my surprise on the 1st of July mtn debited R985 instead of the R150, when I called mtn to query I was told that I have upgraded to a R580 package and was told to call Rewards, Rewards confirmed that my upgrade was canceled on the 26th of May so I need to call mtn to fix my package back to R150. It has been a very bad experience because I have been calling mtn & Rewards but nothing ever gets done. My current bill shows that I will still be charged according to the R580 package which I am not even getting the data. All the consultants says is " I see the error & will escalate so it gets rectified" but nothing is done after that. I've written to mtn complaints department and been waiting for feedback for more than a week when it says I'll get feedback within 48hrs. I really can't wait for August so this contract can end and I'm sure not going to renew it.