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Kindle devices can buy or download to themselves, e-books that are 'in' an Amazon.com account, that they are both
A. logged in to (logged in to that Amazon.com account where the book is)
B. registered to.
I'm not sure if there will not be any problem with registering a Kindle that you ordered in her name, but really, there shouldn't be any problem. Kindles generally come shipped in an un-registered state, of course, so you can register them to any account you like. So I wouldn't worry - unless there's some purchasing option, that you actually used and invoked, that pre-registers the Kindle before shipping! and I don't think there is such a purchasing option, it would involve sharing your password ***** just anyone at Amazon... an unheard-of thing, really.
I think it'll be fine. Just register the Kindle meant for your late wife, to your own Amazon.com account, when you receive the Kindle. That'll see that it all works out fine.
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