I have kind of a similar issue with them.

In December last year I went to Barcelona and rented an opel corsa at the airport. I also rented it with snow chains. At the counter they provided me with a Key and the snow chain kit and they said were the car was parked. Nothing else, so no verification of a kind was done before receiving the car but on the contract there was indication of rim scratches.

During our travel to Andorra, I was always careful, and only used snow chains once.

Unfortunately when I went to deliver the car, at the airport, I was late for my flight so I didn't had enough time to check the car with the assistant.

A few weeks later I've received an email with "supposed" rim scratches that I've done to the car. I knew the car was given to me already with rim scratches, it's on the contract, but now they are saying that the scratches are deeper and it needs to be fixed, so they decided to bill me for the repair. I replied saying that I wasn't aware of new scratches, and I was careful when driving, but they ignored it and so did I.

Today I've received an email from Profaktura Auslandsinkasso GmbH saying that their client (Sixt) contracted them to collect my bill and now i have to pay more money because I didn't pay it on the right amount of time, plus all the expenses from Profaktura Auslandsinkasso. So I'm not sure what I should do. Honestly I think that Sixt is trying to make me pay for all the rim damages that were done before I rented the car.
I was careful, like always, I didn't drive on dangerous roads, most of them were highways so I'm surprised with it.

What should I do? Should I contact Flying Lawyer?