Bad service from vodacom

Hi,my name is Wilson Magondo ID no *** ,i have a contract with vodacom and I recently received a call from vodacom agent saying that they wanna migrate my current contract which is Flexi R230 to R550 I then asked if that'll be done for free of charge because i cant afford to pay extra fees,then she said there wont be any extra charges to my suprise at the end of the month I was charged R1058 instead of R778.when I ask about it now I'm taken from post to pillar over that.now An invoice has been generated for R1113 that I have to pay. Pls help