Upvote:2
Dulles Toll Road is FREE if going to and from airport. It is only toll (w/ 50-year-old coin payment devices) if exit early -- do not proceed all the way to the Airport.
A little hack (not worth the effort) = drive to airport, go around the circle for drop-off's and then return to the toll road and exit as wish .. doing this allows one to skip paying the exit fees.
Since YOU don't be getting off early, just drive to airport. Easy Peasy.
Worse case, go to airport like dropping off a relative/pal/colleauge.. and once pass the airport's front doors, keep going and look for signs for rental car rental lots.
Your rental car company can explain it to you. If you are renting from a tiny auto rental place, call one of the big ones (Hertz, Avis).