When grocery stores in rural Alaska need fresh supplies to restock their shelves, they rely on a novel delivery system -- the U.S. Postal Service.

True to its motto, the Postal Service delivers the goods through snow and ice, using bush pilots who fly as far as 700 miles into the wilderness carrying everything from produce to diapers.

A typical 50-pound package costs a shipper $3.38 in postage. It costs the post office between $40 and $50 to deliver, officials said.

In perhaps the most extraordinary example of parcel post Alaska-style, two years ago a freight company shipped 10,000 concrete blocks and bags of cement from Anchorage to Wainwright, Alaska, 713 miles north. The shipment cost $232,000, but the freight company paid postage of only $34,000.

The loss to the Postal Service: $198,000.

Although new regulations would permit the Postal Service to turn down that shipment today -- on grounds of safety, not cost -- the service continues to lose money in smaller bites on probably every shipment it makes to 265 remote towns, which rely on the post office to deliver virtually every necessity of life.

The Postal Service makes up the losses on its Alaska parcel post by adding about 50 cents to the cost of every parcel post package mailed in the lower 48 states, according to shippers who have intervened against a rate increase pending before the Postal Rate Commission. Postal Service officials agree the cost of delivering parcels in Alaska results in above-cost rates elsewhere.

That's because, by law, the post office must charge the same rates for all packages, no matter where they're mailed, and it must break even on its package delivery system.

The cost of Alaska parcel post is heavily responsible for the Postal Service's average 24 percent rate increase request for parcel post, according to postal officials.

More than the cost of sending a package through the post office is at stake, say the shippers and others. Because parcel post rates in the lower 48 states must subsidize the Alaskan deliveries, the service competes less effectively on price with United Parcel Service -- the nation's principal package delivery system.

The Alaska parcel post service is "particularly insidious because it permits UPS with its billions of parcels a year to charge 50 cents more for each one without having to do anything" and still be slightly below the post office for the type of business it seeks, said Timothy May, managing partner for the law firm of Patton, Boggs and Blow, which represents the Parcel Shippers Association, composed of companies that use either UPS or the Postal Service.

Deputy Postmaster General Michael S. Coughlin said the post office carries such a small fraction of the nation's packages that he doubts there is much impact on UPS prices in any event. The post office last year carried only 121 million pieces of parcel post, Coughlin said. UPS officials said the private carrier logged 2.778 billion packages -- about 95 percent of the market.

For its part, in presentations before the Postal Rate Commission, UPS, which is currently having labor difficulties, strongly defends its pricing policies. "Our prices are based on our costs," said Robert Kendall, an attorney with the Philadelphia firm of Schnader, Harrison, Segal and Lewis. "Our prices are not tied or linked to the Post Office."

Coughlin acknowledged that some shippers would like parcel post rates to be more competitive on price with UPS. "Frankly," said Coughlin, "a lot of smart people have looked at this, and I don't know what the alternative is. We are a national postal service. We have an obligation to provide mail service in Alaska, and this obligation has an effect on our prices."

The Postal Service has two kinds of parcel post facilities in Alaska. One is the familiar post office. The other is the airline freight terminal that bypasses the traditional post office. Although parcel post rates in the lower 48 states are geared to surface transportation, there are no roads to many of the villages served in Alaska and the barge season is short, officials said.

Harvey Goldstein, manager of logistics and distribution systems for the Anchorage division of the post office, said a visitor to the bypass terminal "would see an operator backing a trailer up to a loading dock and a forklift operator lifting pallets onto a floor scale. A postal clerk records the weight and verifies the postage, and the forklift operator moves it to a storage area to be shrink-wrapped and loaded onto a plane. You might see an entire pallet of soda pop, or mixed groceries, or paper towels, or Pampers -- any common groceries."

No longer can there be enormous shipments of concrete as in the 1988 incident in which a freight forwarder used the post office to mail the materials for a small boat harbor and launching facility.

But if concrete shipments are down, grocery shipments from the bypass facility are growing. The facility "saves some handling" for the post office, Goldstein said, even though it costs more to ship by air than by truck.

The point of having a bypass process, according to Coughlin, is to save the Postal Service money by relieving it of the need to handle individual pieces -- such as cartons of milk or boxes of diapers -- inside the post office.

"More people {in Alaska} are becoming aware that it is less expensive to ship things by mail" than by other means, Goldstein said. In 1987, he said, bypass mail out of Anchorage totaled 53.5 million pounds. Last year there was 18 percent more than in 1987 -- 63.2 million pounds. Compared with the same period of 1989, shipments were up another 17 percent through mid-June this year.

Some shippers in the lower 48 states are outraged. "The Postal Service . . . performs the sole function of providing a direct subsidy, transferred from the pockets of other parcel users, and paid directly to Alaskan air carriers for the sole and exclusive benefit of the citizens of Alaska," said David A. Bunn, executive vice president of the Parcel Shippers Association.

"This is not mail, and it certainly is not mail service," Bunn said in testimony before the rate commission.

But the other side of this argument is best expressed by Sen. Ted Stevens (R-Alaska), ranking minority member of the Senate Governmental Affairs Committee's post office subcommittee.

"I agree there is a disparity," he said. "We have an obligation to maintain postal service to the Alaskan rural areas. It is the cost of doing business in Alaska."

"We haven't asked for any special service," he said. "If you want to mail diapers from Maine to Florida, you can. They will go by road. In Alaska there are no roads."

The Alaska parcel post program is the kind of situation that turns up in postgraduate political science seminars as a quintessential public policy issue. It involves politics, economics, transportation, competition -- and socioeconomic questions.

The mail service is important to Alaska for reasons beyond the parcels it carries, for example. Dependable revenue from the Postal Service provides an income for bush pilots, who provide passenger and freight service as well. The rates are regulated by the Transportation Department.

In addition, the recipients of the shipments are, in Stevens's words, "the poorest of the poor, primarily." He said that "80 percent of the people are eligible for federal support."

May, representing the interests of shippers who want to hold down costs of transporting packages, said, "If we want to subsidize these people, there ought to be an appropriation from Congress, rather than a deal made to have the post office do it.

"If these people need it," he said, "and Congress doesn't want them to pay for it, then all the taxpayers ought to pay for it, not just the people using parcel post."