R100 interest for every unsuccesful debit order

I have a contract phone with Vodacom, I reversed the debit order for march because I was having financial challenges . this month they deducted double amount plus R200 more. I called them to understand and the lady who was assisting me told me that everytime when a debit order bounces there is a charge of R100, like this is insane even the banks do not charge that kind of interest could someone please explain to me what the hell is going on before I terminate my contract.