I think in Julie's situation, since she will be going on a trip prior to this one with the
free dining package, she will have already purchased her AP, so that wouldn't work for her. But, what you suggest would work for someone that does not have to have the AP beforehand.
You can buy your AP ahead of time, either via phone or internet through Disney, or at the
Disney Store. Some people like to buy them ahead of time to spread the expense out. Also, if you buy it ahead of time, you can log into the passholder website using your voucher #, and be able to check out the special deals, book AP room-only rates online, etc. You're also "supposed" to have an AP before booking these AP rates over the phone with CRO.
Of course, you can also buy your AP when you get to WDW, just like any other ticket.