I didn't get a reply to this post, so we went ahead and booked with Shuttle2LAX.com. What they do is buy the empty seats from the other shuttle companies then resell them cheap. The price is the same no matter where you are going, but they only service a certain area. It is clearly listed on their website where they serve. It is $20 for the first person, then $11 for each additional passenger.
As soon as I booked online (I think you can only book online?), I got an email telling me that I was assigned to Prime Time Shuttle, giving me Prime Time's info for any questions or problems and telling me where to meet the shuttle.