They are glad to get their mail, send off a package and maybe chat a bit while they still can. In December, the government said it planned to sell it.

“We don’t know what’s going to happen to this beautiful, Depression-era architectural jewel,” said Sara Meric, 87, a retired script analyst who has used this post office since 1959. “If, God forbid, this slaughter does go through, some entity should make sure that this building is protected.”

The Santa Monica post office, with its distinctive PWA Moderne style, is one of about 200 post offices around the country, dozens of them architecturally distinctive buildings, that the Postal Service has indicated it may choose to sell in coming years because of its financial problems.

Eleven historic post offices are already on the market in places like Yankton, S.D.; Gulfport, Miss.; Norwich, Conn.; and Washington.

In many cases the buildings have not only been community hubs, but also remain among the most architecturally distinguished buildings in their towns, legacies of New Deal efforts to put America back to work.

So as the Postal Service tries to shrink, it is often finding itself in battle with groups trying to prevent what the National Trust for Historic Preservation last year labeled one of the most significant threats to the country’s architectural heritage.

“Unless the U.S. Postal Service establishes a clear, consistent process that follows federal preservation law when considering disposal of these buildings, a significant part of the nation’s architectural heritage will be at risk,” the trust said in a citation that placed historic post offices on its most endangered list.

When these post offices close, preservationists say, important public buildings become private preserves as they are refurbished into commercial spaces like high-end retail stores. Though many of the buildings’ exteriors are protected by local landmark laws, many of the interiors are not and developers tend to make changes like renovating lobbies.

In one case, a post office built in 1937 without protected status was demolished. That building, in Virginia Beach, was knocked down in 2009 to make room for a Walgreens, despite attempts to save it.

“There are lots of quite significant post office buildings that are threatened because the Postal Service itself is threatened,” said G. Martin Moeller Jr., a senior curator at the National Building Museum in Washington.

The Postal Service lost $15.9 billion last year, and last month announced it was considering eliminating Saturday letter service. It says that the real estate sales raise revenue and save on operating costs, and that the shuttered post offices are those where the number of customers had declined, or where it found a less-expensive alternate site.

“The trend has been to maintain the post offices that are operational while saving money at the same time,” said Sue Brennan, a spokeswoman for the service. “The Postal Service follows all applicable laws regarding the disposal of historic buildings.”

There are some 31,000 retail postal offices in the country, most of them leased. But roughly a quarter are government-owned and as many as 1,100 were built in the 1930s, when the government hired architects to design civic buildings.

In 2011, the Postal Service identified about 3,700 post offices — some leased, some owned, some historic and some not — that it said may have to close in coming years to save money. When that plan met with opposition, the service announced it would instead try to lower expenses by reducing operating hours.

But even as postal officials agreed not to eliminate outlets in some towns and cities, they did leave open the option to sell valuable properties and relocate services. “Periodic sales of post offices will be ongoing,” Ms. Brennan said.

The agency acknowledges that in recent years the sale of post office buildings has accelerated, and in 2011 it hired CBRE, a commercial real estate services firm, to handle the transactions.

“Our biggest concern is the way they’re going about it isn’t transparent,” said Chris Morris, a senior field officer for the National Trust and project manager for post office buildings. “A lot of us are very confused about the process.”

Advocates say there have been too few public discussions or assurances that prized buildings will be protected.