They could in fact actually open every box to check the contents. 15 million is not some unfathomably large number of boxes per day. It would increase the cost of shipping across the borders that require that additional checking, by whatever amount is necessary to hire and organize staff to do that checking. That increase in cost could be shifted to the senders, and in turn the volume of packages would decrease. Trade would not "shut down," but it would decrease.
That's just from one carrier. I'm amazed someone is arguing that this is a practical task instead of asking if these tariffs are really worthwhile or if there are better ways to do this than mass privacy violations and checking every piece of mail sent in the United States
I don't personally think it's worth it, no. To me, though, that's a different issue than "it can't be done."
"Yes, we can do A. The likely result of this action, however, is X Y Z." I prefer to present the situation like this rather than short-circuiting via "I know we are unwilling to accept X Y Z," and so therefore, "we can't do A."
it's not pedantic. there is a very wide chasm between impractical, which can quickly become practical given an appropriate change of conditions -- see: the entire history of computer development, and impossible.
