Letter writing became a common occupation for Civil War soldiers on both sides. Most had never been very far from home before they enlisted/ were drafted. Now they are going places and seeing things they had never experienced. Yet they still longed to remain connected with what was happening back home. Its natural then that letter writing became an important part of camp life. Not only did soldiers write home, they longed for letters from home. Many letters contain suggestions, even complaints about wishing there was more news from home.
Also done only once a year, say at the "home" event as a special treat. One or more letter writers try to personalize the letters to each reenactor, or the real wives/sweethearts of the reenactors write the letters. Another approach is to write up one or two generic letters which are photocopied and inserted into the individually addressed covers.
Another approach for involving spectators one reenactor group has found helpful: a soldier receiving mail says he has lost his spectacles and asks for volunteers to help him read his letters. He picks a couple of different children from the audience.
8) US Christian Commission envelopes for Union troops, used by soldiers to mail home. A helpful prop to have on hand among the soldiers to illustrate their desire to keep in contact with family. (I got into reproducing patriotic stationery because I first started reproducing USCC stationery for my living history reenactment, see website for details). Also US Sanitary issues covers. I have a few designs reproduced as well as some Sanitary Fair stamps. Feel free to contact me about either of these options.
10) Covers without stamps. There was no "free soldiers' letter". Someone paid the postage. Either the sender or the recipient had to pay to redeem the letter. But when someone was going home or returning from home, often they would carry letters with them. So it would be legitimate to have a returning chaplain or soldier bring back unstamped letters from home to the unit in a mail call.
Please feel free to email, call or snail mail me with questions as well as other ideas you have tried about doing mail calls.