They waive the fuel charges for any minor complaint. The employees are worried about bad feedback, and fuel charges frequently lead to customers leaving a bad rating when they get the survey call the next day.
Just say something like the aux port wasn't working and they'll waive the fuel charges. Don't do it too often though. I'd only do it if there actually was a legitimate complaint about the car, I'd return it empty knowing that there's a good chance they'll waive the fuel charges.