Thanks to a special deal with Uncle Sam, the nation's 650,000 postal workers pay much lower health insurance premiums than their 1.4 million white-collar civil service counterparts.

Even if they buy the same policies, with identical coverage, government workers in Defense, Justice, Agriculture and other departments and agencies pay from $50 to almost $400 more a year in premiums than do postal clerks and letter carriers.

The reason is that the U.S. Postal Service is a government-owned corporation that bargains with its heavily unionized work force over pay and fringe benefits such as health insurance. Eight of every 10 postal employes carries a union card.

One provision of the union contract requires the Postal Service to pick up a bigger chunk of the total health insurance premium than the government does for its other, less organized workers.

Health insurance premiums next year are going up an average of 19 percent. But the paycheck bite will be less for postal employes. For example:

Blue Cross-Blue Shield will raise its high-option premium next year to $117.35 every two weeks. The government will pay $52.29 of that premium for white-collar workers, leaving employes paying $65.06. But the postal service will pay $65.36 for its employes, who will then pay $51.99.

The government next year will pay $52.29 per pay period toward premiums for Aetna's high-option family plan, and workers will pay $50.31. But the Postal Service will pay $65.36 of that policy, leaving its employes with a premium of only $37.24.

Premiums for GEHA, one of the more popular low-priced plans, will go to $71.25 next year. The government will pay $52.29 of that and white-collar employes will pay $18.96. But postal workers buying the same high-option family coverage will pay only $5.89. The Postal Service will pay $65.36 of that total premium.

Workers who buy the Mail Handlers Plan, open to both white-collar and postal employes, will have a monthly premium in 1984 of $72.11 for the high-option family coverage. White-collar employes chosing that plan and option will pay $19.82 while postal workers will pay only $6.75.

And the list goes on.

Within the next few weeks, Congress will consider legislation that would raise almost to Postal Service levels the share of the premium that the government will pay for its white collar workers. But unless and until that happens, white-collar workers will pay more than postal employes.