I purchased one ahead of time through the Sunpass website. My recollection is that I put my personal car's plate number into the "license plate" field, even though I didn't plan to drive down this time. I added the rental car's plate number to my Sunpass account shortly after exiting the MCO garage. (Very weak data signal inside the garage, so couldn't do it in there.) There is an Apple app that you can download for this purpose. There doesn't appear to be an official Android app for Sunpass, though I noticed that the main Sunpass website has a "Mobile" section which I guess works out about the same.