Whoever imports pays the tarriff. If you order something over the internet from someone not in the US, that's the consumer.

There is a lot more direct consumer ordering from international vendors now than there was 20 years ago of course, for obvious reasons.

Note Aug 29th is also the end of the "de minimus" rules for import duties, where a shipment worth less than $800 was exempt from import taxes and duties. Some tariffs and other import taxes have always existed, but that's why you rarely saw them when ordering consumer goods internationally to the USA, if it was worth less than $800 they were skipped. That's going away, you'll be paying import taxes on every international shipment you order directly as a consumer, even if it's a $25 t-shirt -- exactly how you pay these, at what point they are calculated by who (even how to calculate them?), and who invoices you how and when as a consumer -- well that's what nobody including international shippers have figured out yet, which is what the OP is saying means they can't really ship internationally to consumers in the USA for the time being. it's gonna be a clusterfuck.

Turns out maybe there's a reason there aren't usually major changes to whole structure of import taxes made with only months notice, and tweaks and changes to them still being made only weeks/days before implementation, with no real implementation guidance provided?

Both UPS and FedEx have been handling this correctly for years. They provide a simple option where you can choose who pays the tariffs (the shipper or the recipient). If it is the recipient, you just include their email and phone number so they can be contacted.

The “only” difference now is that the $800 limit no longer applies, so every shipment must include this information.

Which basically means end of Temu, Alibaba express, majority of Etsy sellers, etc.