Transworld mail for debt I already paid off

TLDR: Credit card company confirmed that they had ruled the dispute in our favor and we had indeed paid off the $150 bill that the merchant says we haven’t. Car rental merchant has sold debt to Transworld and I just got a letter from them asking for money. What do I do?

My wife rented a car from a company near us for her Uber driver program a couple of months ago. We returned the car earlier than planned and thought that would be it.

But next morning we saw a new charge on the credit card and an email saying the rental has been extended. We called the branch and told them that we didn’t have the car and after a bunch of rude phone calls they acknowledged that they had the key of the car and the return was actually complete. My wife had paid with her credit card so all transactions were finally cleared (only charged for what we used) after a couple of weeks.

Fast forward to about 2 months ago we got a few letters saying we owe the rental company money and our credit card was also charged. We opened a dispute with our credit card after going nowhere with the rental company’s customer care service (they couldn’t give us an itemized bill or invoice on what they were trying to charge us).

The credit card company rules in our favor and went with the itemized bill and saw that we had indeed paid everything off. The rental company however kept saying we didn’t and transferring my call to trans world who had no idea about this amount or debt.

Today I finally got a letter from transworld asking us to pay this $150 debt. Im planning to send them a debt validation letter with the transactions and bills I have along with a statement from my credit card company acknowledging I have paid off all obligations. What else can I do?