Last year I picked up a very nice example of an American Express Co. stock certificate signed by Henry Wells and William Fargo (of Wells Fargo fame). Most of the ones you see are beat up, but this one is in excellent condition. These can sell for relatively big money, although most of the ones you see online at asking prices of $3,000-4,000 are just nuts. They are more common than that... but are always in demand.
As an aside, the circled star geometric cancel on the stamp is frequently misattributed as a postal cancel, i.e., an improper use. I have seen it on several American Express stock certificates.
Well, this week I picked up a nice stock certificate to go along with it, from the American Merchants Union Express Co. Issued in 1868 and signed by William G. Fargo as president.
From Britannica.com:
By the end of the American Civil War, American Express Co. had so flourished, with some 900 offices in 10 states, that it attracted competition in 1866 in the formation of Merchants Union Express Company. For two years the two companies engaged in cutthroat competition and, on the verge of financial exhaustion, finally merged on November 25, 1868, to form the American Merchants Union Express Company, with Fargo succeeding as president. The company was renamed American Express Company in 1873.Both stock certificates are shown below.

