Incorrect bill

I recently upgraded my contract with vodacom and to what we agreed on I was supposed to pay R499 on the Flexi 390 but now I see it has gone up to R539, why?? Can we revert back to the original amount we agreed on I don’t understand why now it went up,I never agreed to any other products, what I agreed to was the Flexi 390 (including handset installment) for R499 as well as the 24months 2GB complimentary, not R539 can this be corrected please