just wanted to mention that we used Fox in SF - it's very popular in CA cities. It was ok - not the best maintained cars in the world, but cheaper and therefore worth it to us.

Does either Hotwire or Priceline do one-way rentals? May be worth a check. If you want to pick up in Palo Alto, you could take Caltrain from SF to PA for MUCH less than a $103 shuttle.