I posted a very similar question in the Hertz forum a couple weeks back

http://www.flyertalk.com/forum/hertz...servation.html

After doing some research, "An $8 per day fee on vehicle rentals at the Philadelphia International Airport will support construction of a facility to be used by customers of rental car companies."

Source: http://www.mwn.com/pubs/xprPubDetail...Detail&pub=382

Trust me in that I have checked EVERY airport rental facility at PHL Airport and all of them have this fee - including Thrifty and Firefly even though they are off-site. Technically the law states that any rental car facility doing business with the airport is affected with this new charge (which means any that have a shuttle, or kiosk at the airport as well).

There is no way to avoid it (at least as far as I know) For a little while priceline and Chase Rewards Mall didn't know about it...so you could have lucked out for a week or two - but that's now been fixed. I also had booked several reservations with Hertz & Thrifty before this fee was even in effect - but they have gone in and updated all of my reservations to reflect this fee now as well.

This fee appears to be permanent. I am now forced to search rental car rates across multiple sites on a daily basis to try and get rates even close to the rates I used to get.