UPU Congress' consensus vote to adopt the compromise plan represented "an historical moment," the agency's chief says.
Delegates will consider what to do in the face of the announcement last year by U.S. President Donald Trump’s administration to withdraw his nation's participation.
The military delivered more than 88,000 tons of packages and letters last year and must now plan for a possible disruption.
For only a second time in its 144-year history, the Swiss-based Universal Postal Union held an "Extraordinary Congress," this time under U.S. pressure to change international postal rates.
The first international organization dates to an 1804 treaty on the Rhine River. In the 20th century, organizations for carrying out a lot of the world's cooperation and development proliferated to find complex solutions and prevent wars.