I'm guessing that all packages will need a customs slip declaring contents and value, and the tax will have to be paid on the value. There will be no more exemption on inspections, but they can't inspect a billion packages. There will be random checks, and I don't know what happens to companies that make false declarations. Do they just start another puppet company to ship from, or do they do the trans-ship tactic and send it to another country that still have the de minimis exemption.
Of course, China could cut a deal. I'm not trying to be political, but Trump does love to make deals.
I don't think anyone, including the US and China, knows what will happen. It will be interesting to see how all of this plays out.