Edgars Instalment is incorrect. Instalment more than doubled.

My Edgars statement instalment prior to 01/08/2020 was R235.41. I made a purchase of R618.00 at Jet at the beginning of August 2020. I received a statement saying that my monthly instalments are now R551.00. I know that this incorrect as how can I instal more that double for a purchase of R618.00. Despite this I still made a payment of R235.41 on 25/09/2020. I called the call centre a few times and even requested a call back and nobody called. In the interim, I will received SMS's saying that my account is overdue. I finally hold of a lady who advised that 2 lines of interest was added on my account which is an error. She advised that she will log a call for the error immediately. This was on Monday, 02/11/2020. I received a call in the same week still advising that my account is overdue. I also made a payment on November for R235.00. I will not accept that my instalment more than doubled, if that was the case, then I should have made a cash purchase. Please can this be rectified urgently. Thanks