When I order things from China to the UK, on AliExpress, they arrive 'delivered duty paid' - i.e. aliexpress collects certain taxes from me at checkout, then the item doesn't get held up at the border.
So there does seem to be some mechanism for closing the buyer-seller-taxman loop. Unfortunately I have yet to find a reliable way to send things using this system.
I was gonna mention that, but I felt I was waffling on a bit, so I deleted it!
We've got the same thing with GST, basically like VAT or sales tax. So that'll appear on the invoice from AliExpress or Steam or wherever.
Businesses have a threshold before they need to charge it though. If they're under that threshold (like a small business), but the value of goods is over another threshold, then the receiver has to pay GST.
If I remember correctly, customs would mail me a letter, and I'd pay it like a tariff. Which brings me back to the main point, that's just that the carrier has nothing to do with it. It's ridiculous to get them involved in a transaction they're not a party to.
Process might be slightly different, I'm remembering from about fifteen years ago.
Using the EU and UK systems is optional, but without using it the small business's customers might be annoyed by the handling fees applied by the recipient's mail carrier, and the delay it causes.
I have seen some foreign merchants (I think DigiKey?) offer the choice, as their business customers don't need to pay VAT directly in this way, and may well prefer to do the import paperwork themselves.
I haven't seen a choice for any large retailer (Amazon, eBay, Etsy, AliExpress etc). They don't want customers annoyed by fees, or returned packages from unpaid fees and duties.
Every business that wants to send something to the UK is required to register with the British government and collect VAT on items shipped to Britain. So yes, the US could have a system like that - just get a couple DOGE kids to vibe code it tomorrow, huh.
Foreign businesses aren't required to register and collect UK VAT on items they send there, but by doing so they avoid their customers paying the £8 handling fee charged by Royal Mail.
An £8 fee makes a cheap product bought from China unappealing, so those sites do pay the fees. It's less important if the British person is buying something for €100 from a tiny French business.
I remember one UK content creator had some recipe books. I think the way to get them to EU was order from Ebay. SO big enough platform to have implemented whole thing... Not sure if that really works.
So there does seem to be some mechanism for closing the buyer-seller-taxman loop. Unfortunately I have yet to find a reliable way to send things using this system.