After a bustling Monday that a postal official in Fairfax called "one of the busiest dates I can remember," many post offices were still crowded yesterday with customers who suddenly found themselves 4 cents short.

The price of a first-class stamp was raised to 29 cents on Sunday, and people were buying either the new transitional "F" stamps at 29 cents apiece or 4-cent "make-up" stamps to use with their old 25-cent stamps.

Extra clerks were assigned, some post offices opened early and closed late, and many stations operated with all service windows open. They said business has been heavy since Saturday, with Monday being the worst.

On Monday, said Marc Reed, customer service manager for the main post office in Fairfax, "The line reached to the outer lobby . . . . We even had folks in the outer lobby selling 4-cent stamps . . . . {The crowd} far exceeded what happened during Christmas."

The Fairfax station opened 15 minutes early Monday and closed 30 minutes late. Reed said he had instructed Fairfax stations this week to open early if there was a line and to stay open late if the crowds continued.

Betty Gunby, a clerk at the Brookland station in Northeast Washington, said that, "Since Saturday the line has been out the door." The station had ordered extra stamps, she said.

At the main post office in Gaithersburg, Postmaster George Glover said the station had maintained its regular hours but that things had been "extremely busy" since the weekend.

He said that on Monday the station sold out of 4-cent stamps, got a fresh supply from Capitol Heights and nearly sold out of the second batch by the end of the day.

At the District's Main Post Office at 900 Brentwood Rd. NE, cars were lined up "30 and 40 deep," requiring security guards to come out and direct traffic, said Elizabeth Carter, spokeswoman for the D.C. post offices.

By midafternoon yesterday, however, the Brentwood station had quieted down considerably and only a handful of people were waiting.

For those who may have unwittingly mailed a letter with the old postage, the fate of the letter depends in part on who receives it.

Art Shealy, a spokesman for the U.S. Postal Service, said letters mailed with insufficient postage are delivered to their destination stamped POSTAGE DUE. It's then up to the receivers to decide if they want to pay the extra postage.

Potomac Electric Power Co., for example, does not accept mail stamped postage due. Chesapeake & Potomac Telephone Co. said that while the phone company has a policy of accepting postage-due mail, the policy could change at any time if it is determined that people are taking advantage of it.

Shealy said that if a 25-cent letter is returned marked postage due, all a person needs to do is affix a 4-cent stamp to the letter and mail it again.

Shealy said that the Postal Service, as a precaution, had printed 8 billion of the transitional F stamps in August last year, most of them here at the Bureau of Engraving and Printing. He said that the stamps had been stored in limestone caves outside of Kansas City, Mo., to keep them fresh.

He added that even though the F stamps carry the warning, "For U.S. addresses only," they were good for all letters sent to overseas U.S. military installations, including those in the Middle East.