Competency and explanantions

I took a Wifi router from Macro Crown mines and with it came a vodacom sim card. In store it was explained to me that the line was open. I received a first invoice of R3904.88 of which after inquiring Vodacom reduced the amount. Then we spoke and Vodacom said they had put a R50 cap on the line and to my surprise I have received a new invoice that indicates a need to pay double the monthly installments which clearly indicates that the cap was not put on the line. Now am having an outstanding bill which I feel is really not fare on my part. Please assist.