Never accept sales form Mondo!!!

I was contacted to upgrade my data contract from R299 to R249, I accepted it and then I cancelled it on the same day. I asked and made sure with Mondo to be put back to my normal contract that I was paying R299 for and that everything will be back to normal as was before the upgrade. I pay for both my phone and data contract R1298 monthly and all of a sudden I get debited R4020 last month and then this month R2888. So in total I have R4300 taken out of my account with no reason. I heard now by Mondo that when you cancel an upgrade all the previous promotions form the previous contract get charged for in full and i get put on a month to month basis, which was never ever explained to me by Mondo. Its been 7 days and no one can resolve this, Mondo said they will investigate but I don't trust them at all. MTN just has not been assisting me and I cant speak to anyone in the billing department. I am furious!!!!