High Intrest rates and sending summon letters

i had discovery membership which i cancelled after i lost my job, there had been an excess amount that i need to pay for which Discovery handed it over to SSDA. i received a call from them for which i acknowledge that the arrears and that same week paid over a thousand rand. I had negotiated to pay R250 every month and on certain months paid more , when a payment is late they bully me by sending final letters of demand and they charge me for each call , email that they make to me. I have received another final letter of demand because my EFT did not go through the irony is that they want me to pay the outstanding balance of R1648.11 including penalties, if i am barely making it to pay R250 how can i pay that amount. Here is my question to the SSDA is there no way to lessen that amount that way i can pay is off sooner?