Update: as @jkp notes below, The decision was reversed less than a day later.
original post
Letters will not be affected.
A lot of supply chains Will not be affected because they don’t use the US Postal Service.
But many individual packages will be held up, including pretty much everything from Temu, Alibaba, aliexpress, and SHEIN, and some Amazon third-party sellers (not the ones that say “shipped from Amazon”).
Also, some Etsy purchases will be affected because some of those are actually front stores for Chinese companies.
no word on how long the delays might be.