I rented a car at Alicante Aiport. I had a voucher for a kombi car that was already paid for before the whole rental. At the airport, the sixt personnel informed me that they don't have exactly the car that I ordered but in exchange, they have another car that is in premium class but with the "special offer" for me the price will be higher by only 20 euros (the guy didn't mention that it is 20eur per day). The whole rental process was really fast and nice but at the end of my 10 days vacation, I was charged an additional 200eur which is actually the same price that I paid in the voucher. Of course, I signed the contract I should read the terms before signing but it was the middle of the night, and also If I have a voucher for a specific car I should get that specific car and it' shouldn't be my problem that they don't have it. Unfortunately, I feel cheated. Even though the rent was smooth and nice but in the end I paid 2x more than I planned.
Edit: I got half of the extra price back from the sixt after e-mail conversation