In theory, most countries are signatories to the Universal Postal Union and World Customs Organization (branches of the UN) to keep the post/customs system humming along.

I tried to read up on their “rules” on this topic and it’s a bunch of wishy-washy hot air other than some standardization of customs declaration forms, and I guess HS codes.

Otherwise the only way you get everyone to agree on something: by getting them to agree on nothing during their junket meetings.