KCork...Remember that you can cancel a car rental reservation at any time without penalty. I'd keep the reservation that you have and keep looking for a better rate.
As far as your hotel stay at the Sheaton, since you live in the Philly area, why don't you just drive to airport and use one of the offsite parking lots. We always use Pacifico. I'm sure there are other's that can recommend other offsite parking lots that work for them. They are all located by the airport auto mall. At Pacifico you pull up and go inside and an attendant drives you and your family in your car to the airport. When you return, you collect your luggage and call them and they pick you up. The attendant drives you back to the lot and you go and pay. You can find a coupon for Pacifico online (just do a google search). If you do that route it's less money than doing the park and fly at the Sheraton.
Rumor has it that the Southwest rates are coming out today. I'll have to see that to believe that.