I just got mine this past weekend when we went down. Either way you need to go to guest relations. If you buy them thru DVC website it goes on your credit card and you need to pay for it when your statement comes. Plus you need to print out the voucher and take it to guest relations when you get there.
If you wait till you get to a park and go to guest relations it will go on your credit card then. So take into consideration. Do you want to pay for it now or later when you go on your trip.