Package deliveries will continue six days per week and post office branches currently open on Saturdays will remain open Saturdays. In addition, delivery to P.O. boxes will continue on Saturdays, the Postal Service said.
The American Postal Worker Union condemned the move, saying service cuts "will weaken the nation's mail system and put it on a path to privatization."
The APWU called the postal service's funding problem a "congressionally mandate financial crisis," caused by enactment in 2006 of the Postal Accountability and Enhancement Act, requiring that the USPS pre-fund 75 years worth of future retiree health benefits by 2016.
"No other entity -- public or private -- bears this burden," APWU President Cliff Guffey in a statement Wednesday. "Since the PAEA took effect in 2007, the Postal Service has been required to pre-pay approximately $5.5 billion per year. Yet the same law prohibits the Postal Service from raising postage rates to cover the cost."
Joseph Beaudoin, president of the National Active and Retired Federal Employees Association also condemned the plan, which he said "would undoubtedly affect the livelihoods of our postal employees."
"We ... believe there are solutions to address our nation's and the Postal Service's finances that are not shouldered by federal employees," he said in a statement.
The USPS had considered dropping both mail and package deliveries on Saturdays, but the plan was altered due to "strong growth in package delivery (14 percent volume increase since 2010) and projections of continued strong package growth," the Post Office said in a statement.
As it is, dropping mail deliveries on Saturdays is expected to save $2 billion per year in operating costs.
The Post Office said the new schedule would be accompanied by attrition and personnel re-assignments. The agency said the public largely understands the need for the postal service to cut costs.
Market research done by a variety of organizations -- survey companies and news outlets -- have found that nearly 70 percent of the public indicates they support the cut back to a five-day delivery schedule, the Post Office said.
"The American public understands the financial challenges of the Postal Service and supports these steps as a responsible and reasonable approach to improving our financial situation," said Postmaster General Patrick Donahoe, who is also the chief executive officer of the service that receives no taxpayer dollars.
The Postal Service has already implemented a cost-saving consolidation plan. Since 2006, the agency has cut $15 billion out of its annual operating costs, shrinking its workforce by 193,000 and merging 200 mail processing locations.
Donahoe said the rise in e-commerce was a major factor in saving six-day package deliveries.
"We can play an increasingly vital role as a delivery provider of choice, and as a driver of growth opportunities for America's businesses," he said.
"The Postal Service should be seeking ways to expand its offerings to the American people so that it can remain relevant in the digital age," Guffey said.