You pay the fare on the bus. It's $6.00 and you need exact change. Follow the signs for ground transportation at Dulles, exiting the lowest level of the main terminal. If you are one of the first 15 - 20 or so people in line you should not have trouble with that amount of luggage -- there are overhead luggage racks. It takes about an hour and is not a fun trip if you aren't among the first to board and don't get a seat, but still worth it when you are on a tight budget.