Credit card paid up letter not received

I am so dissapointed with Discovery Bank. I have made multiple calls requesting a paid up letter for a credit card that I closed and it is still showing as though I am owing. I cannot dispute that on my credit record without a paid up letter and honestly it has been a nightmare. A manager promised to send me a letter yesterday and there was no letter sent. I recently just closed an account and it showed closed the same month and here I am struggling with just getting a simple letter from discovery so I am able to at least dispute it myself since they have failed me. I would not take any credit product anymore with the company.