A mid-19th century British anti-slavery cover achieved $17,000 at Robert A Siegel in New York.
That's double its $7,500 estimate.
The anti-slavery cover was sent between two northern states of the US
The piece was among the stars of an auction of pieces from the Eubank collection on December 13, which was held to benefit the Smithsonian's National Postal Museum.
The cover displays an African family with broken shackles at their feet.
A banner at the top reads: "Haste Happy Day! The time we long to see when every son of Adam shall be free!"
An additional detail shows the man holding a Bible directing readers to Isaiah 58:6: "...loose the bands of wickedness, undo the heavy burdens, let the oppressed go free and break every yoke."
The design was British, but the envelope was designed for use in America - where slavery was still practised.
However, the fact it was sent between two northern states (Vermont and Massachusetts) means that its persuasive power was probably limited.
The cover also displays a rare 1851 3c stamp, further enhancing its rarity.
A used 1851 1c blue imperforate pair realised $12,500.
It's the type Ib variety and displays a distinctive triple transfer at the top left.