Contract wrongly billed

I have upgraded my Vodacom contract at the Goldfield Mall in Welkom in September, the lady who helped me said my contract amount is R229.00 per month and the screen insurance is free for the first year. Suddenly I received a invoice from Vodacom for R347.50 per month. I went to complain immediately and still nothing is been done about it. Vodacom Welkom said they send a email to head office but its not been corrected after 4 weeks.