We rented a car for 4 days using a third party website, which didn't give a clear explanation of the rental conditions, in particular in what refers to insurance coverage and fuel.
When collecting the car it turned out that we had to pay 107 euros for a full tank (even if we knew for sure we were not going to use even half of it) and were informed that the insurance waiver was 900 euros and otherwise we would have to take an overpriced extra insurance for 60 euro (for which they tried to make a very good case by trying to scare us to death about terrible things that happened to previous customers and showing us pictures etc). Luckily we didn't make even the tiniest scratch to the car, but when collecting the car we were told that we were not allowed to claim any money back from the remaining fuel. Which essentially means we gave Goldcar a present of ~70 euro worth of fuel. We were treated horribly by the staff who were prepotent and rude.
We also tried to contact them by phone. Be warned: it is really difficult to get anybody on the phone. We did manage, but only to speak to a very unprofessional person, who picked up the phone saying 'yes?' in an angry tone.
Avoid them if you can.