You should create a Disney Movies Anywhere account. It's free:
https://www.disneymoviesanywhere.com/
The DMA account is what keeps track of all the places you have access to your Disney movies. It also links to their rewards site where you can earn points for purchased movies.
Once you have this account, you go into it and in the settings you link all your purchase accounts: iTunes, Vudu, Microsoft, Amazon, Google. Link as many as you have. Each one you link just means that any Disney movies purchased will show up in all your linked accounts. Purchase/Redeem in iTunes, you see it in your Vudu account for example. Redeem in Amazon and you can download in iTunes for example.
Now, when buying Disney codes you usually have these options:
DMA = Sometimes shown as DMA/DMR. You are buying the movie and the rewards points for that movie. These are typically the most expensive to buy. Usually $1 more than the rest.
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DC+ = Digital Copy Plus. These are redeemed at
http://www.digitalcopyplus.com/. When you do this, once you enter the code you have to select the provider you want to get the movie from. Usually once a provider is selected, the code is no longer valid. But as long as you have linked all your accounts to your DMA account it doesn't matter. This code does NOT contain points if redeemed in iTunes/Amazon/Google.
NOTE: There is a DC+ trick to getting points. If you redeem a DC+ code through Amazon you get points for the movie. This is because the way Amazon does redeem codes. They don't just add the movie to your account.....they basically credit your account with the money to purchase the movie. By doing this, it works just like you are buying the movie (using your credit) and you get the points. As long as Amazon is linked to your DMA account, the movie will be available everywhere you are linked.
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DMR = You are buying just the points tied to a movie. You see these often times for $1 and it will say Avengers 150pts. You will NOT get the movie....just the points.
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XML = This is the old workaround where you "technically" need the disc in your drive to redeem in iTunes. Someone figured out how to trick iTunes by mounting a DMG file with the XML file in it. You download the DMG, mount it, iTunes opens and you redeem the code. You will ALWAYS get an error in iTunes, but the movie will be added to your purchased list.
NOTE: Only Disney XML movies will redeem in HD. Do not buy non-Disney movies in XML unless you want just the SD version. Also, Disney XML versions usually do NOT include any iTunes Extras. You'll get just the movie.
In the end, just make sure you have all your purchase accounts linked to your DMA account and you can buy anywhere and watch everywhere.
-Kevin