Not sure what time they open. Suggest reading reviews. After my recent experience with FLIGHTCAR I will never use them again.

They are supposed to open at 4 (I found out the hard way), but are usually late. The people you talk to on the phone do not communicate with the staff in Boston, so, however many times you confirm with them, expect the desk to be empty when you arrive.
Also, note that the shuttles are only running at 30 minutes interval, so, if you arrive at 6, don't expect to leave there until 6:30 even if you do find someone at the desk right away.

I have the same opinion as Rob. I had such a bad experience with them that I won't use it again.