If you purchase the DDE before your trip you will need to fax or send the DDE form which will require your AP number. But, as YCFAN said as long as you have your voucher that should be ok I'd think as you would just enter that number on the application.
Keep in mind that the DDE card expires one year from the time it's issued so if you submit your form this month it will most likely expire the end of March 2007. I'm going to submit mine in about 2 weeks, so I'm curious to see if it will have a March or April expiration.