Horrified to find I now ow more than TRIPLE the amount originally quoted!

I got into a bumper bashing with a Telesure Group client 3 March 2018. My insurance had just lapsed so I was liable to pay. On 12 April 2018, they emailed me a letter of demand for R 7446,22. I responded to make a payment arrangement to pay the money back - R420 pm for 18 months; last payment due at the end of October 2019. 4 November 2019 I received a call advising that the final amount due was in fact R25005,58 and I owed R17559,36. How is there such a variance? How do I complete an agreed payment term only to be told I am liable for a little more than THREE times the original amount quoted?