A lot of sellers are equally confused. But the buyer pays it. This is a form of tax by the US government and it is not the responsibility of a Japanese proprietor to figure out, and to pay, US taxes. Similarly, it is not the responsibility of shipping agents or companies. Some have opted to not ship any US shipments at all, in response to the tariffs, depending on the country. but it seems to be settling into a pattern of an added cost forwarded onto the buyer by the shipper. That is what happened to me with the shipment from Poland and I owed money to FedEx for it.
for a purchase of mine from Singapore (Laiv Harmony equipment) The inventory was already in the US in a distribution warehouse before the tires were implemented, so I did not need to pay for it. Depending on the company you’re dealing with that may be also there to work for your advantage.