According to the United States Postal Service motto, ''neither snow nor rain, nor heat, nor gloom of night stays these couriers from the swift completion of their appointed rounds.''
But administrative mismanagement may succeed where inclement weather has failed, according to a new 512-page report on the agency.
''The trend in the Postal Service . . . is toward self-destruction,'' argues Ralph Nader, whose Center for the Study of Responsive Law issued the report, breathlessly titled ''The Postal Precipice.''
The Postal Service is under siege from increased private competition in nonletter and overnight-delivery mail, the study notes. And a serious loss of revenue is looming from emerging private electronic transmission of messages and documents. At the same time, the small congressional subsidy for mail delivery is slated to be phased out in 1984.
Individual letter writers have borne the brunt of the Postal Service's efforts to turn itself into a profitmaking enterprise, the study claims. There have been numerous cutbacks of service to individual customers - including shortened window service hours and reduced pickup schedules at collection boxes - while corporate clients are courted, the report charges.
The Postal Service is ''catering to business mailers at the expense of personal correspondence mailers. (There is) a strong bias toward sellers who use the Postal Service commercially,'' Mr. Nader alleges. A Postal Service spokesman said the agency had not received an advance copy of the report and would have no immediate comment on its contents.
Defenders of the Postal Service note that while it may be an appealing target for criticism, by several measures it outperforms mail movers in other nations. For example, the Postal Service handles 110 billion pieces of mail per year, or 161,879 per employee. As a result, the American postal worker is 44 percent more productive than his closest competitor, the Japanese mail worker.
And although it is little consolation to those who miss being able to send a letter for 3 cents, US postal costs compare favorably with those in other countries. The 20-cent cost of mailing a first-class letter in the US is the 12 th lowest of 14 industrialized nations surveyed last summer. Only Belgium, at the equivalent of 19.9 cents a letter, and Switzerland at 19.2 cents, were lower. Germany charges the equivalent of 33 cents.
Finally, the Postal Service has managed to break into the black, after decades of losses. The service ended its 1982 fiscal year with a surplus of $688 million. It also made money in 1979, although its accumulated deficit since 1972 is $4 billion, the study says. Even the Nader report, written by Kathleen Conkey , does not see an immediate collapse of the postal system, but rather fears a continued gradual deterioration.
Over the next 20 years, Mr. Nader predicts, the process of deterioration will reach a point where it cannot be reversed. ''It is not going to grind to a halt tomorrow,'' he adds.
While mismangement by postal officials is a key reason for the service's problems, Congress has also been at fault, the report charges. Since passing the Postal Reorganization Act of 1970, Congress ''has abdicated responsibility for postal problems,'' the report says.
To correct looming challenges, it advocates boosting Postal Service research-and-development expenditures, which in 1981 were $28 million, or a meager one-tenth of 1 percent of sales.
Such spending would help the service work aggressively on development of electronic mail transmission. The report argues that ''if a private computer delivers most of the mail on the block, the remaining letters in the carrier's mailbag will be more expensive and service will decline.''
A 1980 George Washington University report found that 49 percent of today's mail stream is vulnerable to diversion into an electronic form. And most of the volume that could be diverted is first-class mail, the Postal Service's most profitable class.
Congress will have to provide some direction to the Postal Service on electronic mail, the Nader report argues. ''By refusing to offer real direction, Congress has pushed the USPS further behind its competitors.'' Meanwhile, the study charges that ''the Reagan administration has been openly hostile to any viable postal role in telecommunications.''
Congress should also continue the subsidy that is slated to end in 1984, Mr. Nader argues. Continuing to appropriate $2 billion or $3 billion ''would allow continuation of door-to-door delivery'' to homes, among other things, he notes. The Postal Service has ended door-to-door delivery in new housing developments. Nader also advocates forming a Post Office Consumer Action Group to speak for the interests of individual postal consumers.