I am not really sure about getting the slipper put on top of the cake. However, I just came back from Disney on Friday. I had the mini-castle cake and the slipper dessert. I actually thought they charged for it when I ordered it because they take your credit card number. We dined at 1900 Park Fare for my birthday. Our server told us they don't actually charge for it until you show up for your meal. When I looked at my statement, this is exactly what they did.
FYI, they had a 48 hour order deadline to order the cake. When I ordered my cake on the 31st of May, the lady in fine dining said it had changed to 72 hours. Fortunately, the very awesome CM I had, said there was no paperwork or information on the change.
She was very forceful with the lady, and got me my cake order. You may want to check the 48 or 72 hour rule.
It probably has taken effect by now. Hope this helps. By the way, the cake and slipper dessert were delicious.
Missing Disney already.