Above, scan of a Civil War era illustrated envelope. The New York Historical Society's online collection contains hundreds more.
The printed envelope came into use in America in 1840 and was first used for advertising or satirical purposes. In the 1850s to 1860s 'corner cards,' with printing applied in the upper left-hand side of the front of the envelope, were common. By the onset of the Civil War, printed envelopes were already in use as a propaganda medium.Found via BibliOdyssey, which offers a great introductory post with links to other online collections of 19th century envelope art: Link.