They make a mistake and I need to pay

I did an upgrade on 21/12/15. There was problems with the signing of the contract. Nicky constantly waited for someone toe help her check and complete the contract. They showed me the 20 places (really 20) I had to sign, so I did not read through everything.I showed her in the book which phone and special I wanted. On 06/01/16 I did another contract. After the first month my son said he did not receive all his data, but I thought it was because it was not a full month. The next month we thought they swapped the two accounts around. I phoned MTN. They said I had to go to Glenfair. There I spoke to Michelle Manley. She admitted it was a mistake of Nicky, even showed me the booklet of December with the correct contract. She said they could correct it, but instead of R349, I would have to pay R429 because it is a case of he said she said. Really, because they made a mistake, I have to pay more????