WASHINGTON — Already mocked by some as “snail mail,” first-class U.S. mail will slow even more by next spring under plans by the cash-strapped U.S. Postal Service to eliminate more than 250 processing centers. Nearly 30,000 workers would be laid off, too, as the post office struggles to respond to a shift to online communication and bill payments.

The cuts are part of $3 billion in reductions aimed at helping the agency avert bankruptcy next year. They would virtually eliminate the chance for stamped letters to arrive the next day, a change in first-class delivery standards that have been in place since 1971.

The plan technically must await an advisory opinion from the independent Postal Regulatory Commission, slated for next March. But that opinion is nonbinding, and only substantial pressure from Congress, businesses or the public might deter far-reaching cuts.

Many postal customers will be upset.

“The post office is a mainstay of America, and the fact that these services will no longer be available is absolutely crazy,” said Carol Braxton of Naperville, Ill., as she waited in line at a mail sorting center Monday with the holiday shipping season picking up steam.

“Well I’m not happy about them, but what else can you do with this economy? If they’re getting ready to go bankrupt, it’s better to cut back than to go totally bankrupt,” said Deborah Butler of Brandywine, Md., who was at a Washington, D.C., post office. “You still need them. Because everybody can’t afford the other ones, like express mail and things like that. Even though the world is computer literate, everybody doesn’t have computers.”

At a news briefing in Washington, postal vice president David Williams said the post office needs to move quickly to cut costs as it seeks to stem five years of red ink amid steadily declining mail volume. After hitting 98 billion in 2006, first-class mail volume is now at less than 78 billion. It is projected to drop by roughly half by 2020.

The agency already has announced a 1-cent increase in first-class mail to 45 cents beginning Jan. 22.

Williams said in certain narrow situations first-class mail might still be delivered the next day — if, for example, newspapers, magazines or other bulk mailers are able to meet new, tighter deadlines and drop off shipments directly at the processing centers that remain open.