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.
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.
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.
https://taxsummaries.pwc.com/united-kingdom/corporate/other-...
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.
Searching "EU IOSS UK" also shows some sort of support from Shopify and similar.