On the contrary, when renting a car a couple of years ago I had following situation - I booked a car and paid, then I arrived at the airport, and at the counter I was told:
* 5EUR/day more because I'm below 25yo
* 5EUR/day more because it's a diesel car ("so I don't want diesel" - "but we have only diesel")
* gas policy is full/empty, so I have to pay for gas in advance; I check the contract and it says I have to pay "market rate", I check online and market rate is 20% less than what they want me to pay, I say this is not market rate, they say it includes surcharge for fueling the car.
In the end I didn't have much choice and rented the car, but the experience was much worse than buying air tickets.