Being charged more than contract I signed up for

When I upgraded my contract I signed up for a new contract that should cost R619pm. I just got my first bill and was charged R784.50 in stead of R619. I went to the store where I did the upgrade and a very nice lady logged a complaint for me to fix the error. The reference for the error is USD ***. <br> <br> My problem is that it apparently takes a month for billing queries to be resolved. I am now expected to pay the full amount and MTN will credit me next month. This is unacceptable to me. I budgeted for R619pm not for R784pm. Why must I pay more than the contract I signed up for just because MTN cannot get their billing correct? This is now the second time this has happened to me? Why can't MTN bill me according to the deal I signed up for? <br> <br> Please therefore immediately correct my account to only charge me R619pm. I am not a bank and cannot be expected nor can I afford to fund MTN's poor billing system. <br> <br>