You can sign up for it now, just in case you might need it. No problem if you decide to use the rental car after all. And yes, you can call a day before to add it.How close to my vacation to WDW can I book ME? I have a rental car set up, but if I change my mind, can I just call ME the day before my flight and sign us up for the bus? It would be four of us. This is for Christmas night. Thanks for any information.