Incorrect amount debited to my account

Cell C informed me that I was due to upgrade my cellphone contract in September 2014. I went on the 24th of Sept to do my upgrading in Vredenburg (+-220 Km from our town) My current contract amount was R249 per month, but my new contract was R239 per month. What a surprise when my debit order was presented as R299 per month. I called customer care - no joy. They said I must go back to the store where I signed for my new contract. Well, back again to the store - still no joy. They said this will be sorted, but then I saw the amount of R299 was deducted again. I send a mail to complaints department, but they did not came back to me. So for 3 months I paid R60 rand too much.... Nobody told me that if I do an upgrading that my amount will increase even if I sign a contract for a lessor amount. I think it is not fair that I had to pay without knowing for what I paying for.