I reserved an "economy" car, but was given a Mercedes Benz with less than 5,000 km on it. Awesome car - but then was charged additional insurance for it.
I called a week before rental to confirm deposit amount so that I would have enough room available on credit card. Also inquired about any additional fees/deposits required at check-in (this was a rental abroad). When I showed up, the safety deposit was nearly 2x as much as was quoted, and 4x the rental price. Also, a deposit was required for toll roads - an additional $100 had to be reserved. Luckily, a friend had an extra card for emergencies and we were able to get the car. We returned the car outside office hours without a problem, but freaked when the final charge came in a week later. The "itemized" receipt was a list of codes that I couldn't decipher. I emailed the billing department (and heard back a week later), the codes were for insurances we had declined (but were applied because we were given a Mercedes), tolls, a parking ticket we never got, and the administrative fees for ticket. THE ADMINISTRATIVE FEES WERE THE SAME COST AS THE ALLEGED PARKING TICKET. I'm still trying to get proof of this parking ticket but haven't heard back. Currently disputing charges with credit card. I'LL NEVER RENT WITH SIXT AGAIN.