Great American Finance review: I would like to have my paid in full letter mail to my home address immediately.

I paid my account in full with Great American Finance Company 8/31/22. I didn’t receive a paid in full letter as promised. I reached out to this company in September, told I had to wait until my payment cleared from my bank, letter would be generated 30 days from my payment date. I explain my final payment was made using my debit card. I again contacted this company in October, was told paid in full letter would be emailed to me. I have not received my letter to this date. I tried calling today 10/18/23, my call went to directly to voice mail. I called on my house phone, call went through by was immediately disconnected by the representative. Please explain why interest is accumulating on a closed account paid in full?