The Trump administration has threatened to pull out of the Universal Postal Union, the United Nations-run regulatory agency for international mail, on the grounds that it benefits China at the expense of the U.S.

But a major obstacle to Trump carrying out the threat is that the administration would then need to negotiate 191 separate bilateral deals with constituent nations to replace it.

“If the U.S. pulls out of the UPU it then has to negotiate bilateral agreements with every single country," said Kate Muth, executive director of the International Mailers Advisory Group.

There’s no actual legal reason why the U.S. has to be part of the UPU, a system that dates back the late 19th century, postal experts note. The UPU is simply a convenience, an agree-upon system of rules and standards for the international community. But its benefits would become apparent if the administration pulls out.

The administration threatened to do exactly that on Wednesday, claiming that UPU’s rules allow China and other countries to ship packages of up to 4.4 lbs at the same rate for flat mail. Officials estimated that this rate has amounted to a $300 million annual subsidy for China. The pullout will become effective in one year if negotiations for better rates fail.

Practically speaking, if the U.S. did pull out of the UPU, it would just set up its own rate system. “You’d move to a system of self-declared rates negotiated on a bilateral — or even a multilateral — basis," said Michael Short, spokesman for the National Association of Manufacturers. "This basically already exists for anything shipped over 4.4lbs.”

It would be an enormous amount of hassle to get there, said Jim Sauber, chief of staff for the National Association of Letter Carriers. “It is grossly inefficient to saddle the State Department with negotiating all of these individual agreements and losing access to the UPU structure,” he said.

White House officials appear to agree that it would be easier to stick with the existing system. "The strategy of this administration is to get to a point where we can self-determine rates," one official told reporters Wednesday. "We would prefer to stay within the UPU. It does provide benefits."

Observers note that there is no reason why the Trump administration has to pull out of the UPU to negotiate different deals. Nothing prevents the U.S. — or any other country — from striking its own separate deal with other countries on postal matters. Doing so wouldn't violate the UPU’s rules, and the U.S. has several existing side agreements already. It had one with China that expired in June.

But the administration has struggled to progress in talks with China recently. White House officials conceded Wednesday that talks between the U.S. and China have stalled, with no end to the impasse in sight. “I’m very cynical because we have given our ask and China does not respond," said White House economic adviser Larry Kudlow. "I worry that they are moving in the wrong direction."