I rented a car from Avis last week. The final bill had a random $23.74 fueling charge on it. I emailed them about it with a picture of my gas receipt and they apologized and took off the charge. No explanation of where the random charge came from. I'm sure they do that systematically and just keep the charges that people don't notice or dispute.
Also, if you drive your car less than 50 miles, Avis automatically adds a $16 fueling fee. You can Google about it. I guess the idea is that as long as you drive less than that, it costs you a flat $16. This is totally separate from all their fueling plans. Even if you fill the car up, and even if you tell the attendant, you will get the fee added. You have to go to the counter and wait in line and ask them to remove the fuel charge. How many people are eager to get on their plane and drop the car off and never realize they paid a $16 fueling charge for nothing? A lot I'm sure.
The van I rented had 52,000 miles. I've never seen a rental with that many miles on it. It had hail damage too.