I looked at the two different types of rentals (prices subject to change, of course), but here's what I found when I searched for your scenario
(flying into PHL airport on June 4th at 2:40pm. leaving the evening of June 19th and returning back to PHL the morning of the 21st. Flying home again July 17th - returning on the 19th. Returning home the late afternoon of July 31st. ) *Note: links open in new windows
With Avis, a long term rate structure for a 58 day rental was much cheaper than doing three separate rentals.
But, you will not be allowed to park the rental at Avis when you fly home those two times. You would have to go through airport parking -- I checked and there are two options -- garage parking is at least $20 per day and has direct access to the terminals via a covered pedestrian bridge or Economy Parking is at least $11 per day - it is on a lot on the airport with 24 hour shuttle service to the terminal.
(there is no discount code available for Alaska Airways FF members)
With Enterprise, the rate structure for a 58 day rental would be cheaper than three separate rentals.
Monthly car rentals are your cheapest rate option for your situation. (and you would have to use the airport parking options mentioned above, when you fly home in between.
Always check all options in order to find the best rates!
Hope this helps and good luck!!
Pages from this site, relating to this topic:
Long Term Car Rentals