^ just gotta worry about putting a bare sheet against pages of a book, considering humidity and resulting transfer. Optimally, you'll want to isolate the sheet in something waterproof, like a ziplock bag. If you can put that in between a book, it will be fine. Even if there's a gap in the pages.
Mail handlers don't go opening packages and rifling through books. If the package isn't suspicious (aka: fluid leaking from the package, oddly shaped package, wires protruding from package, ticking sound in package..lol), there's absolutely no reason it should be opened.
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