If you order the tickets for anything other than a UPH prior to your arrival at the resort, they will charge you for the price of the ticket and mail a regular ticket to you. Even if you bring this ticket and your receipt to the front desk when you check in, it cannot be encoded onto your ID. Advantage to ordering in advance is the price discount.
If you wait to purchase the ticket until you check in, then anything that you buy at the resort can be encoded onto your ID. Disadvantage is that you will pay the higher gate price.