Scenario in NZ is somewhat similar. Everything imported or sold in NZ attracts a 12.5% GST (similar to VAT in the UK). Now if you import $1,000 of widgets from China, you have to pay the GST before it's cleared. But there is naturally a threshold where the tax isn't worth collecting, and most Aliexpress / Temu packets fell in that category, so no tax was paid on them. This means the stuff imported by local shops was taxed, but stuff bought from a Chinese (or any other foreign web page) wasn't. Temu gets an unfair advantage over local retailers, and the Govt misses out on tax revenue.
Solution was that if an overseas company gets over a certain threshold of sales to NZ, (maybe $100,000?) then they have to register with the NZ Tax Dept, and collect that tax at time of sale. Now some small time web site that only does the occasional small order to NZ still slips though the cracks, but that's a legit loophole. The BIG online stores that are getting thousands of orders a day basically comply, and collect the tax at "checkout".
I guess the "stick" is that if Temu doesn't play the game, then their parcels get held up in Customs until they clear the due taxes.
Temu might not WANT to do this with the US market, because adding the tax makes their widgets and assorted tat more expensive to buy. But there is zero reason they can't. They already have this sort of function enabled for NZ clients.