When UPS charged me a $684 tariff on $355 of vintage computer parts

Somewhat clickbait. The $684 duty charge was in error, and they got it reduced to the correct amount (an order of magnitude lower)

Not a clickbait, because it was quite difficult to get it reduced, so it is good to know how to do it.

Moreover, the money have not been returned yet, that could take a long time and for many people losing access to such an amount would be very inconvenient, even if they knew that the money would be returned, eventually.

It's definitely good to know how to do it. This guy shares his anecdote and it's worth pointing out while UPS made an error there is plenty of responsibility that can be taken by the buyer and seller when importing. Don't put your life and finances in the hands of someone else like a chump if you can avoid it. The buyer can request/urge the seller to properly declare items (e.g. as used vintage computer parts), and the seller can declare items and prices clearly. The buyer can also pre-clear with customs (this is more advanced, level 2, but Customs has been investing a lot into making pre-clear process easy and useful)