Unissued paid up letter

I gave the red star because their treatment has been horrible. I’m very dissatisfied.I had an account with HomeChoice and it was handed over to Nimble Group. We made arrangements and I finished paying the account. I have been requesting for a paid up letter to no avail. Instead they keep on adding interest which is R87 as it stands although I made the full last payment in May and waited on the paid up letter as they promised.