Postage was free for military personnel serving in a war zone, as I recall. I found a website with postage rates in history:
They say 6 cents a letter in 1968, and that sounds about right.
Politically, keep in mind all that went on surrounding the Vietnam War. Kennedy, the hope of a generation, had been recently assassinated. Lyndon B. Johnson (D) was in office for part of the time; Richard Nixon (R) during the later years, resigning in August of 1974.
It was a pivotal time that, combined simultaneously with the Civil Rights and Women's Liberation movements, Kent State student shootings, Mai Lai Massacre, along with the Watergate Break in, triggered the mindset of the American public to shift from trusting in government and authority, to angry suspicion and flag-burning and draft-card and bra burning, questioning and protesting of government and authority. Martin Luther King, Jr. and Bobby Kennedy were assassinated in 1968. This was all jumbled together, all this trauma just another brick in the wall. Draft dodgers moved to Canada or out of the country to avoid being drafted and forced to fight a war they didn't believe in.
And don't forget the juxtaposition of the Apollo 11 moon landing in 1969... a small shred of belief in the future. We grasped at science, and science fiction. The movie 2001 came out and it was revolutionary in special effects and scope. Star Trek was in it's first TV run, and a loyal and enthusiastic science fiction fan base was born.
Society was polarized, and in many ways still hasn't recovered. Men (no women - women were nurses only) who chose to go to war were called baby-killers. Draft dodgers were called cowards and unpatriotic. There was no middle ground. You either supported the war or you didn't. You either trusted the government or you didn't.
Vietnam veterans took a large share of the brunt of this civil anger because they were lumped together enmasse, demonized by some as mindless soldiers acting complacent with the atrocities of the Mai Lai Massacre and other actions, lauded by others as true and loyal patriots protecting their country's freedom (from the encroaching evil of the USSR... this was during the Cold War.)
Keep in mind the coverage of the war was nothing like it is today. It sometimes took a month to get a letter or an update; television reported on events that had been over days or weeks ago. There was no "on the spot" reporting at that time. Many of the citizen massacres did not begin to be made public until toward the end of the war.
The theme was very much about how the evil Russians would take over if we did not stop them. The war may have taken place in Vietnam, but because Russia was allied with North Vietnam and the US with the South, it was about stopping the "Ruskies" before they took over Asia.
Walter Cronkite was the voice of reason as a news anchor for CBS news. The day he finally publically came out against the war, Nixon told his staff, "If we've lost Cronkite, we've lost the war." Shortly after that, Nixon called an end to the war and withdrew troops from Vietnam.
This is all off my memory, so you might want to fact check for exact details, but it might give you a flavor and some topics to research. Look toward the music of the time... it will reveal a lot about what was going on. The Beatles, Crosby Stills and Nash... there was a lot of change happening in society. Drugs began to be a factor - Berkley California was the epicenter of the hippie movement. Love beads, pot, LSD, Timothy Leary...bell bottoms (Say what you will, I LOVED bell bottoms.)
There were no: video games (except pinball in the arcades), home computers, cell or mobile phones (hard wired into the wall, Ma Bell owned the monopoly), VCRs, CDs, hand calculators, music cassettes. There were three television network stations: NBC, CBS, and ABC and no cable stations. Not everyone had color television. Seeing the NBC peacock in color was a big deal "Come look! It's the NBC peacock!" Music we bought was usually on an LP (ooo! Stereo sound!) or a 45 if you wanted only the single.
If you were a young person at the time, we all knew a friend or a brother who had been called up to the draft. We feared for our loved ones. So many didn't come back alive. We wore commemorative bracelets in support of the MIAs. "War is not healthy for children or other living things" became one of my favorite sayings. Black light posters, Peter Max artwork, Keep On Truckin'.
By the way, it's only now - 40 years later - that people are realizing these Vietnam vets came home with PTSD. About two years ago, a Vietnam vet in my home community murdered an Asian family, and it's my belief his mental state was fueled in part by PTSD from the war.
[This message has been edited by Elan (edited September 03, 2010).]