Charged a penalty fee of R400.70 for a once off returned debit order of R198

Hellopeter, Telkom charged me a penalty fee of R400.70 for a once off returned bebit order. I have a data subscription with Telkom for R198 and I have been paying them, unfortunately for me on the 31 of August my debit order bounced and I called them to ask for their banking details since my incame was late that month..on the 22nd of September I paid for my September subscription and to avoid another bouncing debit order due to late income, I made an advance payment on the 23rd of September for October and all was well and they didn't debit my account end of September since I paid in advance.To my surprise on the 31st of October they tried to debit my account for R400.70 so I quickly reversed the debit order and instantly transferred the balance which is suppose to be R198 and when I called in to query the amount, they tell me that it is a penalty fee of the August returned debit order and they threatened to blacklist my if I don't pay their absurd penalty fee and they will also charge me a penalty fee for remaining month on my contract if I cancel my contract !...I am furious and heart broken, I am bul**** and don't know what else to do , is it even legal for them to charge such an absurd amount for a once off returned debit order???