Incorrect billing on my account

I ordered 2 sets of bedding (Stephanie and Julia) November 2014, when the order it someone else's order.I called HC, to complain about wrong order and they said I should mail it back. I returned it then asked when ill receive my order, they said they'll call me and confirm if I still want to continue with original order or change it. After 3 weeks they called, I told them im only ordering 1 Stephanie. The order came and I received the invoice. January I called them because I wanted to paid off before starting with repayments end of Feb. They told me my balance was R5000+ instead of R2000+-. We agreed it was a mistake and they sent through a dispute form for me to complete so they can freeze account. I completed the form and sent it back 30 January. Since then until now they say they haven't received my completed form. I resent it 2-3 times and sent messages querying progress of dispute, countless calling and they transfer me and the phone never gets picked up. The admin says they haven't received my emails but they are responding to them. Now they say I must start paying end of Feb based on the R5000+- which is an error from their dispatch in the first place.