ONE HIGHLY effective way to cut the budget is to shift costs of public services to the people who use them directly.The cost of the first-class stamp ought to cover the cost of delivering the letter. This principle, we acknowledge, needs to be handled with some care. Whenever the postal rates come into question, you will be forcefully reminded of the magazines that would collapse if they had to bear the full cost of getting them to the subscriber. Like all of the other interesting budget quarrels, this one involves a great deal more than mere money. The concept of charging the user is not one that we would advise Mr. Reagan to follow out the window, but let see at least where it leads. You can reasonably support certain postal subsidies, for instance, without necessarily agreeing that they ought to add up to the present $1.8 billion a year.

The budget contains $22 billion for transportation, just about all of it subsidies of one sort or another for people who want to get themselves or their freight from here to there. Some of these benefits -- the advancement of highway safety, for example -- touch so many people that it hardly makes much difference whether the highway user or the general taxpayer carries the load. They are both the same person. But what about private aircraft? If they paid their full share for the air traffic system -- as, incidentally, the commercial airlines do -- that would reduce the federal deficit nearly $1 billion. The suggestion will be greeted with shrieks from the general aviation lobby. Many a corporate jet would be relegated to the hangar, and a great pall would settle over the industry. Aircraft workers would be thrown out of jobs. That would be tough luck, but if it's a choice between food stamps and private aircraft, food stamps may conceivably have a better claim on the federal dollar.

How much money does the country want to spend running empty Amtrak trains? It makes sense to keep passenger trains on the few main routes where they are being used. Beyond that, the only argument for the trains is to encourage fuel conservation -- and the next generation of jetliners will be more efficient than locomotives.

Putting the cost of the inland waterways onto the barges, for which they are maintained, would cut the deficit perhaps a billion dollars. It would mean higher barge freight rates. And should the costs of harbor-dredging fall on the general public, or on the shippers? That's another half-billion dollars a year.

For reasons that are now wholly historical, the federal government subsidizes electric power in many parts of the country. There's a case for the Tennessee Valley Authority as a model of development and conservation policy. But surely there's none for the gigantic expansion of generating power that it's now got under way. Nor is it clear that the federal government should continue to subsidize the tradition of extremely low power rates in the Pacific Northwest. It's even less clear the the government should continue offering loans, through the Rural Electrification Administration, at an interest rate of 2 percent.

Most of these commercial and regional subsidies have been around for years. Few could be cut without wrenching effects on vulnerable industries and the people who work for them. Each case will differ from the next, and each deserves to be debated on its own terms.But once the country has decided to balance the federal budget -- and that was the command delivered at the last election -- the subsidies all become competitors with each other.The dollar subsidizing barge rates competes with the dollar subsidizing hospital care for the impoverished elderly. The dollar subsidizing cheap power competes with the dollar subsidizing school lunches. In making up your mind about the subsidies, it's useful to keep the nature of his compeition clearly in mind.