When I first purchased it in early December, I bought it at a ticket window at one of the parks. If the ticket windows are not busy, it's a good place to purchase the pass. Sometimes. Guest Relations can get backed up with long lines and lengthy transactions. I believe that all of the same forms of payment are accepted as used when buying tickets at a ticket window.You can wait to buy it then. You can also buy it at the concierge desk at your resort before heading out to the park. Or at Guest Services at the park entrances.
If you purchase at the Express check in points you must use a credit card. Any other location can process a payment using your MB
A PP mentioned that the payment system at the check in points was out of order during their trip. I guess that is always possible and sort of hard to plan for one way or another. The safe bet would be to handle it before you need it. Unless you can potentially run in to an issue of it expiring before your trip is over. Then you want to wait to purchase till you plan to use it