There will likely be more specific advice forthcoming from others, but generally keep such items away from light and moisture/humidity. For paper such as that, I'd think there could be a risk of crumbling if handled and bent frequently, so you may want to place such items in acid-free, clear holders together with rigid, acid-free backers (particularly if there is no writing on the other side) to prevent flexing of the notes/letters. A good example would be the mylar bags and backer boards that are frequently used to store items such as comic books.
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