You can book your own DME, you don't need to bother with a second party doing it. This from the Disney web site....
Pre-Arrival
Read this important information regarding booking Disney’s Magical Express and packing for your vacation.
Reserve Disney's Magical Express with the booking agent for your Walt Disney World hotel reservation or when you make your Disney Resort hotel reservation online or over the phone.
U.S. and Canada Reservations: (407) 939-1936
We have often called the 407 number and set up DME, just have your flight information and reservation number available.