Refusal to refund penalty

I recently paid my outstanding balance on the app. (As was communicated on the app). I then returned the debit order, because the account was already paid. I was charged a penalty of R100.88 because the debit order was returned. I logged a dispute *** 0n 31 Dec. I called the "helpdesk" where I was told they can not refund the penalty as I was supposed to inform Vodacom that I will be paying on the app. Nowhere on the app it is stipulated that I need to inform Vodacom that I am changing my payment method. Please refund the penalty charge of R100.88