The Air Force and the Agriculture Department are going to use special federal stamps in selected field offices this fiscal year as part of a U.S. Postal Service test to see if agencies are underpaying -- or overpaying -- their postage bills.

Most federal agencies send the Postal Service their best guess of their postal costs each quarter, based on a sample of the mail going out of offices. Some mail is in preprinted envelopes, other mail is stamped by postal meter or sent out under bulk rates.

By using the special stamps and comparing the results with previous estimates, the Postal Service hopes to find out how much the Air Force and Agriculture Department spend. "We don't really know how much a federal agency owes," said USPS official Jim Stanford. "We can't say mail costs are undercounted, really. They could be overcounted."

The new stamp will come in denominations of 1 cent, 4 cents, 13 cents, 17 cents, 20 cents, $1 and $5.

"The uncertainty in determining official mail costs is not a very businesslike approach to running either the Postal Service or the federal agency," Stanford said. "It doesn't encourage efficient management." In fiscal 1981, the most recent year tallied by the Postal Service, the federal government spent $760 million on its mail. Because of two postage rate increases since then, Stanford said it is likely that the total could hit $1 billion in fiscal 1983.