The Screen Door at BW had a few bottles of champagne but I didn't notice the label. We wanted plastic wine glasses but didn't find any - or plastic flutes. We saw a pair of "collectible" glass flutes not far from the grocery section that were pricey. I don't remember the exact price, but it was the kind of thing that you pick up, look at the price on the bottom and think, "No way". I didn't see any cake toppers, but you could call and ask. Disney would probably prefer to sell you the topper with cake under it.
I don't know if they'd store anything for you. I sort of doubt it. You could ask, but I doubt they'd do for any extended time.
DisFlan