ROBERT G. BROWN runs the sort of government agency even Ronald Reagan could love: For every 30 cents it costs to run, Brown's agency returns $1 to the Treasury.

And, while the overall federal deficit is growing, fiscal 1987 promises to be a record year for Brown. His agency's sales are expected to hit $32 million, up from $28.6 million in fiscal 1986.

It's a tribute to the nation's booming stamp sales.

They've made Brown, a 46-year-old former letter carrier from Alexandria and former postmaster of Fort Belvoir, the nation's biggest stamp dealer.

In a nondescript one-story brick building in a Northern Virginia office park, Brown runs the Postal Service's Philatelic Sales Division.

It's a computerized marketing operation that has mailing lists with 1.7 million names and handles more than 700,000 letters and stamp orders a year. Since 1982, virtually all mail orders for stamps and postal stationery from collectors and dealers have been funneled through the Merrifield office and its staff of 110.

Brown says his soaring sales are products of the Postal Service's brighter, more colorful stamps, higher postal rates, more sophisticated marketing and a policy of accepting credit cards. The average stamp order now totals $49, an increase of about $10 since the service left Waterside Mall in Washington for the Virginia suburbs.

That's only a portion of the stamp business handled by the Merrifield office. It also sells and cancels first-day covers by the millions and runs specialized sales of new issues to various mailing lists and stamp clubs formed by the Postal Service's marketing division.

"We try to be the place to go when you can't get it anywhere else," says Brown. Actually, Merrifield is only the first stop for a collector's stamp order.

While clerks process stamp collectors' requests on the first floor, most of the stamp orders actually are filled by another 50 employees who work in an office buried in a limestone cave in Kansas City, Mo.

That office, located seven-tenths of a mile underground, is linked by computer with Merrifield and typically mails stamps to collectors 10 days after the order arrives at the Virginia office, Brown says. By having the orders mailed from mid-America, the Postal Service figures it gets the stamps into most collectors' hands a day quicker.

To a collector, the first floor at Merrifield can be disappointing. The only stamps there are on posters on the walls. For stamps by the millions, head for the basement, where the action has been since 1985.

That's when the Merrifield workers took over the task of canceling first day covers for virtually all of the nation's new stamps, work that used to be farmed out to temporary employees hired at postal offices across the country.

The trouble with that service was that it was costly and inefficient, according to Brown. With 50 new stamps appearing each year, Brown says, "We found we were reinventing the wheel 50 times a year."

It took too much time and money to train temporary workers at each new first-day city on how to cancel the new stamps, Brown says. Worse, the service was flooded with complaints by collectors that their covers were arriving double canceled, backstamped, smudged or otherwise damaged.

Today, armed with an armada of canceling devices from rubber handstamps to a specially modified Pitney-Bowes Model D canceling machine, individual workers at Merrifield can turn out 15,000 to 20,000 first-day covers an hour. Spoilage is held to a minimum thanks in part to a modified Doboy-brand wrapping machine that slips each envelope inside a cellophane wrapper to insure the collector that his cover arrives spotless.

The stamps, held in two vaults in the basement, are all "select" quality, the best-centered and best-printed stamps that the Bureau of Printing and Engraving can offer.

Until the service came out with the sheets of 50 wildlife stamps earlier this month, the record for a first-day cover was 12.1 million covers for the similar 1982 issue of the 50 state birds and flowers.

Brown says he has no doubt that the wildlife sheet will surpass that. One company alone has notified Brown's agency that it will bring 7 million wildlife covers to Merrifield to be canceled.

So many envelopes are expected that some first-day cancellations this summer have had to be shifted back to the actual first-day cities.

Brown says his staff is not resting on its upward sales curve. He says the service may soon begin experimenting with telephone sales and open a "postal store" selling items such as albums and framed stamps.

Bill McAllister is a member of The Post's national staff.