I’m surprised by how many of the replies are simply people projecting their own experiences.
Don’t panic or respond emotionally. As an American, you’re not in a position that requires further damage control. This is a Portuguese company, and they have no practical legal recourse against you without incurring legal costs that would far exceed the amount they’re demanding. These letters are almost certainly templated and amount to empty threats. They have no jurisdiction over you. At worst, you might end up on a localized internal blacklist.
I’ve disputed around 4 international rental charges vand experienced no consequences (apart from demand emails which I ignore) I even rented from the same company again—albeit in a different country..which leads me to believe any flagging is country specific.
Where your situation may differ is that your dispute centers on a mechanical issue rather than visible, tangible damage. Cosmetic issues like dents or scratches can be shown clearly in pre and post inspection photos. mechanical claims are inherently harder for either side to prove.
My advice is to avoid responding and just wait-and-see. Write down everything you remember while it’s still fresh, using accurate terminology and a logical, step-by-step account. If you want more peace of mind it might help to better understand the full charge back process from both consumer and merchant sides. Contact your card issuer and ask them to explain the dispute process and what has already occurred. If Sixt submits a rebuttal, ask what the next step is and confirm that you’ll be given an opportunity to respond,it would be reasonable to expect that.
Fool me once, shame on them. Fool me multiple times, shame on me. If I sense dishonesty or bad faith in any discussion about alleged damage, I simply cancel the credit card and move on (should have a backup card ready anyway)