As mentioned what you are buying online are AP vouchers, not the actual APs. You can either have the vouchers mailed to you or pick them up at Guest Services.
The vouchers have no expiration date so they are a great way to protect against future price increases. There have been reports of people redeeming vouchers 5 years after they purchased them. The AP start date would be the day you redeem the voucher for the actual AP and is good for 366 days from that date (the AP is good through the end of the expiration day)