It was mailed to them in a regular mail envelope, albeit an insured one, delivered by the US Postal Service on a routine run from New York City to Washington D.C. The museum has the original wrapper postmarked November 8, 1958 complete with the mail franks and postal comments. The postage was $145.29 ($2.44 postage and the remainder, insurance for $1 million). Can you imagine sending something like this by mail today?
It's really the small things of history that I enjoy the most.
Image courtesy of the Smithsonian Snapshot.