Standard Bank Mobile - Top Up deception

I've emailed your customer care but received no response from anybody. I have a Standard Bank Mobile Sim which is on the Top Up option. On your website it says that Top Up is "Pay now, use later.No spend balance. Top up when airtime runs out". Last month I received an invoice for an additional R444.57 for data usage. I didn't top up the amount. I'm requesting that this amount be reversed as I didn't authorise the amount or my account to be changed to the Flexi Plan. Should Standard Mobile fail to reverse this I will approach ICASA's Ombudsmen for remedy.