What do you know about mail service in the US Civil War?

I think that every service member and family member left back home has appreciated mail service especially while deployed. Mail wasn’t as common in the Revolutionary period. But by 1861 many were familiar and appreciated sending and receiving letters. In modern wars service men and women frequently write letters to be mailed at the death. In the civil war this was fairly common and many dead soldiers had letters stuffed into their pockets to be mailed after their death.
"The U.S. Post Office Department introduced several improvements during the war which made it easier to send and receive mail. Since soldiers sometimes had trouble acquiring postage stamps. If they did get them, they had trouble keeping the gummed bits of paper from congealing into sodden lumps. Soldiers were allowed to mail letters without stamps beginning in July 1861 by writing “Soldier’s Letter” on the envelope; postage was collected from the recipient. In July 1863, postage rates were simplified and in some cases lowered when distance-based letter rate categories were eliminated and all letters given the lowest rate. That same month, free home delivery of mail was introduced in the nation’s largest cities.
To prevent the fraudulent use of the large quantity of U.S. postage stamps held by postmasters in the seceded states, the U.S. Post Office Department redesigned its postage stamps soon after it suspended mail service to the South. The newly designed stamps were distributed to postmasters and customers beginning in August 1861, in exchange for the old ones. Initially Postmasters were instructed to give customers six days following notification in which to exchange old stamps for new ones, after which time the old ones were demonetized (rendered valueless). But the time limit was stretched in some cases to accommodate customers. In New York City, citizens were given about six weeks to exchange their postage stamps. As the war progressed, coins, which were more highly valued than paper money, gradually disappeared from the marketplace. By the summer of 1862, the lack of coinage posed a serious hardship to trade. Merchants began issuing their own promissory notes, called “shinplasters,” and many people began using postage stamps as small change. Unfortunately, shinplasters were often redeemable only where received, and stamps were liable to crumple and clump together."
Pictures: Group in front of post office tent; Mail wagon for headquarters, Army of the Potomac, at Falmouth, Virginia, March 1863; U.S. Mail wagon, 2nd Corps, Army of the Potomac, ca. 1864. The clover painted on the side of the wagon was the insignia of the 2nd Corps: red for the 1st Division, white for the 2nd Division, and blue for the 3rd Division; Distant view of Belle Plain Landing, Virginia, on the James River, with U.S. Mail wagon, 2nd Corps, Army of the Potomac, in foreground, 1864.