Not sure if this is what you are asking but, in the US, The biggest difference is that for domestic mail, law enforcement needs a search warrant signed off by a judge before they can open your mail, meaning that they have to have probable cause that you’ve committed a crime and the evidence of that is in the mail before they can open your mail.
An international package has no such protection and like when a human crosses the boarder, customs, BCP, etc can open any package and inspect it for any reason whatsoever.