I do think that customs policies should be regulated by the UN. It can be a nightmare flying country to country sometimes, and trying to understand all of the customs policies. The United States sometimes is a nightmare to get back into. I think the UN could really smooth this process out.
No, customs policies should not be regulated by the UN, because each state should be able to decide for themselves what kind of customs they want to have. Other nations whose interests are adverse to ours might try to make it hard for us to import or export, or to welcome people the way we want to.
While the United Nations is essentially a trade group for countries, it should not have such a granular focus. Its member nations are still sovereign, and they reserve the exclusive right to determine what does and does not pass through their borders. That is something that countries should never surrender to an outside authority.
I do not believe it is necessary for the UN to regulate customs policies. Customs are something that need to be handled by the individual countries without interference from a higher authority. I do not see why the UN would get involved in this area and I feel like regulating customs would bog the UN down.