When to buy Annual Pass

[Chloesmom]

I am buying an Annual Pass for my US trip in 2 weeks. I used the pass to get a discount on the room. Sooo... do I buy the pass now online or wait til I get there to buy it? Do I get a voucher to print out if I buy online or do they send me an actual pass? Does the pass expire a year from when I buy it or activate it? Thank you for your help!