I'm not sure about pre-purchasing...I have never heard of anyone doing that. But that's a great idea if they'll let you do that at a cheaper cost!
UK brides get the hi-res cd/dvd included in the package because of the high cost of postage for shipping photos...it ends up being a losing proposition all the way around.
ETA: and I see they upped the price after three years. For us, at the three year mark, it's $99.
we bought ours before the wedding. the cost was suppose to be 1k but i think with discounts qwe got it for 800 bucks. we never questioned it but we didn't pay 1k. We used both disney photo and video though.