Welcome to being US citizens.

Anything purchased outside of the US that you import (yes buying parts from a foreign website qualifies you as an importer) may or may not be subject to import duties. There are many different classification codes that apply to bicycle parts. Some have a 0% tariff. Others can be as high as 20% or more. The listings of tariffs can be easily found by searching for US harmonized Tariff Codes.

When you order from a foreign company that company just simply ships to you. The shipper then determines how the item will be sent into the next country. US Customs requires there to be an agent that acts on the importer's behalf (that's you) when a shipment comes into the US. Sometimes the shipper just contracts and picks one to act on your behalf. If not they have to represent you directly or your shipment would have to wait for you to travel to the point of entry and represent yourself.

These representatives have to sit there and based on the information provided on the invoice that was sent with the shipment determine what tariff codes would apply and then agree on what duties should be assessed as a result of their findings.

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