The problem was that I wanted to use a debit card to pay for my car (I got rid of all my credit cards a few years back when I paid off the last of my debts). I rent cars all the time with a debit card and I'm used to rental agencies taking a small deposit (usually $250) on the card, or, in some extreme cases, requesting my Social Security Number and doing a credit check on me.
But Fox had a genuinely idiotic policy for debit-card payers: they needed a copy of my Southwest flight itinerary. It wasn't enough for me to show them the itinerary on my laptop's screen. They couldn't take it on a USB stick and print it themselves. I either had to take a taxi to a Kinko's and print it and come back or call Southwest and have them fax the itinerary to Fox.
An itinerary proves exactly nothing, of course. You can make any itinerary you want on your computer and print it off. Given that Fox got this reservation from Southwest, anything I printed for them was totally redundant. The clerk kept repeating that he needed "proof" that I was flying out of Oakland again. I offered to let him photograph my screen with his cameraphone, but for some reason, that isn't "proof," while going to a print-shop and screen-dumping to a printer would be.
I spoke to a supervisor (who told me that this was my fault for not noticing this bizarre requirement in the five screens' worth of fine print in their reservation "agreement"), called Fox corporate HQ and left a message, then turned around, walked up to the Hertz counter with my debit card and drove off in one of their cars five minutes later.
Update: Brendan sez, "I recently did a little googling about Fox Rent-A-Car in preparation for a trip to California. It turns out they're listed with the Better Business Bureau for bad business practices. The Attorney General of California seems to have some interest in them too. Maybe it's a good thing they made it so difficult to rent."
Update 2: Doug did some digging into Fox: "I followed one of the links to the California Attourney General's site and found that there was a judgment in place against Fox Rentals for various business practices which were illegal under California law.
"What made the hair on the back of my neck stand up however, was the statement on the Attourney General's Website that Fox routinely tracks their customers by GPS. They have used this to illegally charge their customers for driving out of a three state area, but hey, they could certainly use it for other purposes."