vendor is responsible to fill out custom forms, ask vendor
Cross border shipping is the trigger for this, like say selling second hand, seller/vendor might not care about international sales due to this sort of extra work. Some delivery services actively hide this information from the recipient just to make it that little bit more annoying (cant even refer to the original package).
When I want a quick answer, look up a same/similar part on Digikey. They have the HS code right there in the data.