Overcharged for the past 3 months

A sales representative phoned me to do an upgrade. I only upgraded the package and not a phone as it was cheaper as what i was paying at that moment. The deal was 350min anytime minutes, 500mb of data and 50sms all for R369. When the invoice for August came it was R200 more than what the guy said i would pay. So I phoned customer services and said they would fix it. September invoice came same issue. Phoned customer services spend an hour on the phone said it would be fix. So yesterday I received my invoice for October, same thing. So instead of paying R429 a month like I use to I am paying R600-700 a month now. I am also being charge for short messages despite that it clearly states that I get 50sms each month. I am also charged for calls despite the fact that i have over a R1000 worth of airtime carried forward. If this is not sorted out within the next week I am cancelling my contract.