If the government's biggest service agency -- the U.S. Postal Service -- shrinks drastically, taxpayers who have already shelled out nearly half a billion dollars in benefits and retraining for private workers may be hit with a staggering new bill. That is the gloomy -- but many believe realistic -- assessment of one of the most outspoken postal union leaders.

James J. La Penta of the Mailhandlers Union believes that the electronic revolution is here, and that within the next decade it could throw half the postal work force (more than 300,000 employes) out of work. He talked about the problem, and its price tag this week before the House Postal Operations subcommittee.

La Penta believes the USPS will get only asmall share of the pie when the nation's businesses shift over to the electronic transfer of funds and messages. Uncle Sam now has a monopoly on first-class mail. But even that has been eroded by other services, and by the public shift from the letter to the telephone for much its its personal communications.

Within the next few years, La Penta feels, private business, which accounts for 8 of every 10 letters now carried by the USPS will be sending messages, money and invoices electronically rather than through the mail service.

The White House currently has a study going that will ultimately recommend what share, if any. the USPS should have in electronic communications. Close watchers get mixed signals. But La Penta predicts that the president and Congress will urge that the business be turned over entirely to private industry.

La Penta says the nation has been generous and understanding when when private, and semiprivate employes are thrown out of work. He said railroad workers affected by Amtrak were entitled to 100 percent of their regular pay for up to six years, retention of fringe benefits (insurance, retirement), and payments for retraining and relocation, if necessary.

Workers displaced by cheap imports resulting from tariff changes get paid benefits, salary retention and training money from the government. And most recently, the Redwood National Park Act gives full pay, retention of fringe benefits and preferential hiring to employes in the lumber business who lose their jobs as the result of the expansion of national parkland.

Postal workers, La Penta says, already are an "endangered" species, and the government had better begin planning how it will help thousands of workers readjust when they no longer have enough mail to deliver.

If La Penta is correct (and many rival union leaders say he is unnecessarily pessimistic) the days of the jolly postal clerk handling letters, or the neighborhood mailman (or woman) stuffing messages in your box or apartment slot may be numbered.

La Penta says that no-layoffs clauses in the current postal contract will not save jobs since the USPS can and will reduce the work force through attrition. And he doubts that any job protection would be condoned by taxpayers or Congress if there is no work for the postal service to perform.

"We're not talking about black boxes in everybody's house for receiving mail," La Penta said. "That may be years off. It may never happen. But individual use of the mail is relatively small. Business uses the mail. If we lose business letters, newspapers and advertising, a very, very small work force could handle actual person-to-person mail."

La Penta would be delighted if he turns out to be wrong. It could be that the Usps/ will grab a big chunk of the electronic transfer business, and that could mean more income and better jobs for workers. But if he is right, people ought to be thinking about the possibility of a shrunken postal service.

Federal Times columnist Bob Williams, perhaps the best media observer of the postal situation. Believes Congress had better take the problem seriously. He says that La Penta has an "uncanny knack" for predicting what the future holds for the postal service and postal worers.