Linn's Stamp News has a neat article by Labron Harris about envelopes sent within the Confederate States during the American Civil War. Due to shipping blockades, paper became very scare and envelopes were made from whatever was available, including wallpaper, previously used envelopes, and advertising leaflets as shown in the example above.
Of additional interest on the envelopes are the stamps. The Confederacy printed its own stamps as it did not consider previous issues of stamps valid for mailing. However, they didn't print stamps until later into the war. Consequentially, some post masters ended up printing their own stamps for delivery. What a fascinating window into American history these old letters are! Do check out the original article in its entirety to see the other examples.