Paid Up letter needed

I am so frustrated with the level of unprofessionalism and incompetence at MBD legal. I settled my account in August yet they are still harassing me everyday demanding a payment, they say all my payments are not reflecting on their system and I honestly don’t know how this is my problem since I have been paying and sending POPs every month post payment. This is affecting my credit score and is causing unnecessary depression. I need a paid up letter so my name can be cleared!