Poor customer service on closing of an account

On 2020-03-13, I paid my Markhams account in full. I send an email with the proof of payment with a request to close my account and provide me with a paid-up letter, to TFG customer service centre. About two months later, after another email and about four calls to the customer service centre, my account was closed. Up to date (2020-08-19), I did not receive any paid-up letter. I send about five emails and made 8 to 10 calls to the customer service centre and every time they promise me that they will send the letter within 24 to 48 hours. I need this letter to proof that I do not have any outstanding debt, but to no avail. I am very very disappointed in TFG. I was a customer for about 15 years but I will never open an account with them again.