You're a bit late for all that. By the Civil War envelopes were in common use [one New York company alone was making (100 employees folding them by hand) 200,000 envelopes a day in 1855!] And stamps, paid for by the sender, were introduced in 1847; at first it was voluntary, and mail with insufficient (or no) postage would still be delivered "collect." But, on April 1, 1855, pre-payment became
mandatory; after that date "postage due" was double the pre-paid rate. In 1863, however, soldiers were allowed to simply mark his envelope "soldier mail," and the recipient would pay the normal rate.
At the time of the introduction of stamps, postal rates still varied by distance traveled: under 300 miles, letters cost 5¢ per 1/2 oz; over 300 miles, letters cost 10¢ per 1/2 oz. These rates fluctuated in the decade that followed, but the tendency was always downward in cost, and upward in distance. With the increasing use of envelopes, and both paper and envelopes of varying sizes, cost was calculated by weight rather than number of sheets: 3¢ per 1/2 oz, reduced to 2¢ in Oct. 1863. Also in 1863, limited home delivery was introduced in 45 cities across the Union -- general home delivery developed slowly from the 1890s on (not until the 1950s in some areas).
There is a very complete (128 pages) exhibit online:
"Special Routes Across the Lines During the ACW" (lots of illustrations and specific examples).
See also:
https://stamps.org/userfiles/file/symposium/presentations/CharlesPaper.pdf
https://wikipedia.org/wiki/Postage_stamps_and_postal_history_of_the_United_States
https://wikipedia.org/wiki/History_of_United_States_postage_rates