Hi Jennifer! Thanks so much for your question. Disney's Magical Express is a great service, however, it is reserved for those guests arriving or departing Walt Disney World from Orlando International Airport by airplane. Unfortunately, you won't be able to use it for a ride to the airport to pick up a rental car. You could either arrange a taxi or a private shuttle through a company like Mears.
However, there is a better, and less costly, alternative to doing that. There is a car rental facility on Walt Disney World property at the
AAA Car Care Center. Click the link I've provided to see which companies you can rent from and details on picking up your vehicle, etc. We've used them several times and have found it to be a very convenient method of renting a vehicle for just a portion of our stay. I highly recommend this service.
I hope you find this information helpful, Jennifer. Have a magical day, and please let me know if I can answer any other questions before your Walt Disney World vacation!
Kristen