I have a R79 contract with Telkom which they debited as per contract

I have a R79 contract with Telkom which they debited as per contract. Today boom!! the 13th during the month they just decided to deduct R400 from my account, the last money I had for my child’s transportation till month end. When I called to ask what’s the debit for? They said it’s penalty that they didn’t get the R79 on the 26th they got it on the 27th, R400!? Like 5x the actual monthly payment. I’m thanks. I’m cancelling the contract. I don’t mind paying the penalty which is understandable I caused an inconvenience but R400 for R79 never I’m done.