It's all speculation, but I wonder if he also wants to add custom charges to discourage people from importing.
Ultimately the decision international people have to make when dealing with the US is can we afford the cost plus shipping and import duty. In a way even if import duty is not a factor, this just means US people doing the same since right now it's cheaper to ship to the US than it is to have something shipped out.
It may make some people stop shipping to the US in the short term, but ultimately it will be up to US people if the new charges/rules are prohibitive or not.
I guess we'll wait and see. It's a little concerning for me if we're Brexiting as well that it might complicate yet another shipping arrangement, but unless it becomes unrealistic and stupid to do it, I'd like to maintain shipping overseas to as many places as possible for as long as I can.
Spoiler
Right now we're increasingly short of an actual government, as people keep getting kicked out or quitting, which, although entertaining drama, isn't really answering any of the big questions that are going to kick in for private individuals on things like international postage when Brexit hits.