Ridiculously high interest and non existent communication charges

I have an account which was handed over to SDDA for collection. The account was a loan taken with Capitec Bank. My problem with SSDA was their failure to tell me about interest that they have apparently charged on the account as well as communication charges, which mostly are non existent. When I was called by a consultant at SDDA regarding the account I was only told of the amount owing to Capitec which I acknowledged and made arrangements to pay. When I requested a settlement letter in May, only then did they make me aware of the ridiculous interest they have charged on the account as well as communication charges. I have no money to pay for that interest and feel that amount is way too high. My rights are being violated and I need assistance with this matter.