Nick Hamilton swung his 35-pound bag over his shoulder and began pushing a cart filled with large sacks toward the door. It was 8:47 yesterday morning and the temperature already was inching toward the 90s. As he headed outside, he passed a poster that issued a peremptory command: "CARRIERS ARE YOU READY FOR WORK?" the words spelled out. "Your appearance Speaks for Yourself and the U.S. Postal Service."

There was nothing wrong with Nick Hamilton's appearance that the towel he stuffed in his bag to wipe off his sweat wouldn't improve, and he was more than ready for work. Since arriving at the postal branch office at 5 o'clock, he already had sorted out the 1,800 letters and 500 magazines he was about to deliver. He had cleaned up his area around the mailbox delivery and registered mail, and was setting out on his rounds just as he has from that same station for the last 11 years.

He likes his job, he was saying. You're on your own, and you don't have anyone standing over your shoulder. He also likes the exercise and the fresh air, even if it gets too hot or too cold at times. And the pay's good: he's making $15,500 a year carrying the mail. Yet he's seen disturbing changes. The volume of mail, bothletters and magazines, obviously is dropping. It makes you wonder, he says, if people are turning to other means of communications, or what? But the biggest change comes from personal attitudes, both the public's and the employee's.

"You'll find people, particularly older ones, will complain because the service to some extent has deteriorated," he says. "Overall service is fairly good and some of the guys take pride in their work and do a good job. But the service has deteriorated a little bit.

"What's happening? To me, the young carriers just don't have the attitude that the older carriers have. The young fellows here are not quite as dedicated as the older ones. Some of the guys just don't care. I feel that in dealing with the public, the carriers should be a little more courteous. After all, these people are paying our salaries. They're paying us good money, and most people, it seems, feel they're not getting good service. And they have a legitimate gripe."

Authorities will dispute Nick Hamilton about service deteriorating, but they won't deny that the Postal Service faces a severe crisis. It's been six years this month since the new Postal Service emerged, independent and free of its political past, bringing bright hopes for increased efficiency and financial self-sufficiency. The promise has not been fulfilled.

When it inherited the mail service from the old Post Office Department, the U.S. Postal Service had some 715,000 employees, earning an average wage of $10,187. A first-clas mail stamp cost 6 cents. Today, when about 55,000 employees have been cut from the payrolls through attrition, average salaries have risen to $17,327. The price of a first-class stamp now stands at 13 cents, and the Postal Service is asking for an increase to 16 cents by early next year. A "citizen" stamp would be sold for 13 cents. Accompanying that increase would be an elimination of Saturday mail deliveries.

The price goes up, the service goes down, the deficits keep increasing.

In fiscal 1973 the Postal Service lost $13 million. The following year the losses were up to $438 million.In 1975 the figure had doubled to $989 million. Last year, the service lost $1.2 billion. None of this was supposed to happen. When the law creating the Postal Service was signed early in the Nixon years it carried a clear mandate: The Postal Service was supposed to pay for itself after 1984.

What lies behind this financial tangle is not a political scandal in the old-fashioned sense of sticky fingers and grafters. The Postal Service's problems stand at the center of a philosophical debate about the role of government, what it can do and what it shouldn't. It's one heard often in Washington these days when Jimmy Carter seems to be following the Jeffersonian adage that government governs best when it governs least - a theory, by the way, that Jefferson propounded before being elected but never put into practice as president.

The point about the Postal Service, like so many other government services, is not only who's willing to pay the bill? It's who's defining what's in the best public interest, and by what standard is it measured? The real question about the postal Service is: does the federal government have a responsibility to provide decent mail service at a reasonable cost to the consuming public, even if it means subsidizing the service?

At 43, Benjamin Franklin Bailer exudes a sunny disposition and a conviction about his job. He's been Postmaster General for two years now, coming to that job from big business - the oil business in Texas, vice president of international operations, American Can Co.

"There's a feeling that because the Postal Service is a government service, it doesn't have to be self-supporting," he says, "that our government ought to be able to do better by us even if our private businesses can't. Well, the fact that it's a government operation does not insulate if from the economic realities or life. It as our employee unions would be quick to point doesn't spare us from the ravages of inflation, out. In my opinion, part of the beauty of the tough decisions be faced. The law says, in effect, run this thing consistent with what the public needs and make the rate payer keep it self-supporting.

"People say, 'Well, why should the Postal Service be self-supporting? The Defense Department isn't, the outfit that runs the lighthouses isn't. Well, the fact is the Postal Service is generally a business service - the vast amount of mail is moved for commercial operations, and that's different than the Defense Department and lighthouses, as such. It's gotta be paid for some way."

Looking ahead, Bailer sees, in his term, some "disquieting" trends. The hand delivery of material is declining and private companies are expanding their moves into electronic message systems of communications, raising the prospect of machine-delivered mail.

"We used to have all kinds of services at our homes - groceries and milk and ice and medical," Bailar says. "Now about the only services brought into the home are newspapers and mail.

"As our living habits and our ways of communicating with each other change, it's going to change the role of the Postal Service in our lives. The American public, I don't think, wants to, or can afford to, support an anachronistic service, if it comes to that."

Some people see the future clearly. They see a time, not far away, when mail service as we know it will be obsolete, when the Nick Hamiltons will be replaced by new electronic systems. Cheaper, faster and failsafe you see. Even now there's an experiment going on in Japan that's being watched around the world. In a town outside Tokyo, 3,000 households have been wired for a close-circuit TV system through which messages can be transmitted and hard-copy printouts obtained by people.

"It's got some profound implications for traditional postal services," says our Postmaster General. "It's also got some rather disturbing Big Brother elements. You've got to wonder if you want to be that well wired into some central communications system."

Or maybe we do. After all, 1984 isn't that far away.