Onair tv contract

My name is Mmolane and a representative from onair tv called me last month to offer me dstv streama.She explained the product and I agreed to pay an amount of R100 only to realise that once the product was delivered,and amount of R199 was debited even though this was not the agreed amount in terms of our conversation.They didn't even wait for a message to authorise the debit order mandate as per the contract.I sent them messages informing them about misrepresentation by their employee to cancel the contract to no avail.Kindly request your office to intervene and help me cancel the contract.