We use the 0% everytime we make a package purchase.
In order to really be able to use the 0% is if you have no other balance on the card what-so-ever.
As PP stated if you do have any purchases balance on the card, when you go to make the payment it will pay off your "purchases" balance.
If you are able to use the card soley for WDW package purchase and not make any other charges to the card for 6 months than you are good to go.
but I have found a "LOOPHOLE" so to say.
If you have a balance on your card, other than your WDW package purchase, you can still pay directly and soley to the package purchase ONLY if you are within 30 days of the promotional 0% is set to expire. Basically at the 5 month mark.
On 2 seperate times within the last year, I have made payments soley to the vacation package purchase, while still having a regular "purchases" balance, at the 30 days from promo 0% period to expire.
So, it can be done. It's very tricky, you have to plan well and known their own rules. You may have to talk to a few people until you get somebody who understands what you are asking for, but it def can be done.