Sorry.. but for a long time the us did not pay the requested membership fee to the un, did that change recently?
Also.. the us benefits in form of less war
, possibilities to penalize a country with an embargo and lots of treaties as well as world wide trade.
World wide trate is not possible without political fundamentation.
Sure, the trade is handled due to the WTO activity in the end, but not possible without UN.
And UN military operations. If no UN the US would have gone to some places without UN resolutions, causing them to have less support from other countries, less authority and harder job to get money from other nations for the costs.