Getting overcharged and incorrect billing

On 30 October 2015 I upgraded my contract from MTN My Anytime 100 Topup to My MTNChoice 50Mins and I would pay R389 for the Z5 Compact. Although I wanted to keep my old package I was told that they are no longer offered so I have to take the MTNChoice as it's similar so I was told. Upgrade went well, I set my Subscriber Usage Limit to R0.0 (reflected on my contract). on the 30th of November I checked my invoices and to my surprise the amount due was R542 not the R389 that I signed for. My woes didn't end there on the 4th of November I recieved the 30GB free bundles that was meant to expire on the 4th of December and on Saturday the 26th about 10GB of it was still available but while I was downloading yesterday I got message stating that I have used my Usage Limit , I call MTN the lady tells me I have incurred about R176 of my usage limit & my data expired on Monday which doesn't make sense since 30 days hadn't passed. Went to the MTN shop and got told I will be charged R1244 of my Usage Limit. Now my service has been blocked, I got overcharged and will paying for something I didn't sign up for and didn't even use. I have been using airtime throughout the month. MTN is robbing me