I think you can even call same-day to request an ECV, or at least request it for the next day. But that request is subject to availability at that point. I second the idea to contact a few vendors and ask about cancellation policy -- then reserve it now and cancel if you won't need it. Otherwise you may end up needing one but not able to get one easily. I don't know how often that happens but suspect it's a strong possibility during busy park seasons.
I hope your foot heals and you have a wonderful vacation!