My colleague reports that she used this service successfully a few days ago and, to my surprise, actually reports having had a pleasant experience! The shuttle worked exactly as described on the website, no tips or fees were solicited from her, and she made it to San Diego significantly faster than she would have if she had gone the bus-to-Amtrak route.
The shuttle requires booking and currently only travel agencies can do that. Notice "Travel Agency Login" on the upper right corner in the web site.
The Hotmail address listed on the website took her ticket number and confirmed the shuttle reservation on a direct booking with TK, so booking one's ticket with an agency seems to be unnecessary.