Being billed R100 more then signed for in contract

Good Day<br> <br> I took out a contract in May 2015 for the mtn store at the glen across from telkom, sales person was very nice and help and explained everything to us, I to the contract for the Lg Beat the on in the booklet for R299 per month but i want the safer top up version which would be R370 per month which was fine. We signed for everything and everything was sorted, this moth i get a bill for R478 i get a statement but it doesn't explain the billing in detail, why am i getting billed R100 more then agreed upon on the contract and why does it take a week to get a reply on facebook