Do you personally have to purchase them the day before you arrive? We purchased ours last week even though we are going in July because we wanted to be able to link them to our MB and do FP+. We simply linked the code on the vouchers to our
MDE account and will activate them at the ticket booth the day we arrive at our first park. I'm not sure how the process works for offsite visitors, except that they can book FP 30 days in advance rather than 60. The only way to do FP+ ahead of time is to purchase your tickets ahead of time as well.