Originally Posted by flyer703

Yes, and that (or Uber) is definitely the option I would use. My company used to use Boston Coach (https://bostoncorporatecoach.com/reservations/) but I'm sure you can find others. I'd expect ~$150 for a car service or $50-75 for Uber depending on time of day. Either way should be stress-free.
As others have said, Metro is an option but why do it? You're going to be coming off a long international flight, picking up bags, clearing CBP, etc. = TIRED. Then checking in at DCA and going thru TSA. I'd make the transfer as simple and quick as possible. Perhaps you can even catch a nap on the way