On 1 JUN 1861, the United States Postal Service ceased all operations in the seceded states. The Confederate Postal Service took over all the existing USA post offices within the CSA. I've found an interesting web page on Confederate Postage Stamps
, authored by Dr. John L. Kimbrough.
Here's one example of a "cover":
There are over a dozen other covers and adhesive stamps that you can view, on the site. The ones that I think are most fascinating are the "Adversity Covers".
Quoting from Dr. Kimbrough's CSA philately web page:
"As the war dragged on, the South began to experience severe shortages including paper for envelopes and letters. This led to the phenomenon known as ADVERSITY COVERS. Envelopes would be refolded and used a second time (known as Turned Covers). Envelopes were made out of whatever paper was handy. We see envelopes made from book pages, printed forms, ledger sheets, old letters, maps -- the list of paper items used to make Confederate envelopes can be as long as your imagination."
Here's an example of a "adversity cover":
It's made out of wallpaper!
I haven't read the entire site, but there are lots of fascinating details and technicalities that stamp collectors will no doubt love.
I hope you enjoy the site!