SuperShuttle and Supreme both operate from the Shared Van Service desks at Dulles Airport - from the baggage claim area go down the ramp at door 2 or door 6 and you will see their desks. No need for a reservation from Dulles - but you need to make a reservation for a ride back to Dulles. Both take credit cards - and both offer door-to-door service, just like a taxi but about half the price for one person.