If the weather is lousy, look for one of the rental car shuttles, they go right past the FedEx ramp on their way to the pax terminal. Tip the driver a buck or two, you can usually get them to stop. If you are unfamiliar with the DC metro, it can be just a little tricky going from IAD to DCA. After you get on the flyer at IAD (bring some cash, it makes life easier), you have to take the blue line to the orange line (or vise versa). Either way you have to change trains at Roslyn Station. I will second the notion that it takes a good hour and a half to get from IAD to DCA. I used to give myself at least 2 hours from an arrival at DCA to a time when I could report at IAD.
Hope it helps.
Take care,
Fish