I don't exactly know the answer to this, but I will share what I do know. I just read the fine print on this because I decided to use my Discover card this month to make a payment on my WDW vacation for the same discount. The fine print says firstly, for purchases up to $800 and also that the purchase must be credited in the appropriate category. I am also waiting to see if it will show up as "Travel" rather than perhaps "Services" since the money is going to the Walt Disney Travel Company. Sometimes, the categories things end up seem a bit weird.
Anyway, good luck and enjoy your trip.