Nothing will go back to China, but my understanding is all of these packages will arrive with customs fees we'll all need to pay to get the goods. USPS hasn't been regularly charging customs fees for ages because they are undrstaffed to handle the volume.
I expect a lot of packages (I saw that millions (!!) arrive from China daily) will hit US shores and rot for ages while customs processes them. Maybe they'll cave futher because just based on volume, it's unsustainable. For starters, where would they store the stuff?
DHL has been doing this for the last few weeks apparently, also new for them.
Meanwhile, China is retaliating and they are used to putting their citizens through pain, way more than we are. It's a lose / lose.
We're in the "find out" stage. Good luck, everybody, the pain has just begun.