unhappy customer

I have an account with Finchoice , I paid my last instalment on the 29th December 2020, after they sent me a settlement amount on that day . they have referred me to their debt collectors saying I did not pay . I sent proof of payment on the 7th Jan 2021 . Kept on getting calls that I did not pay .When I call they told me they are in a backlog of emails .They deducted another 593 on top of what I paid . today when I call I am told that they cannot assist me until the get the proof of payment . they are still in a backlog .