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If you are importing from a wholesaler or manufacturer, the responsibility for Customs documentation and import duties is one of the things you agree upon when negotiating the shipping terms. Most of the time it will be your responsibility to organize Customs clearance and the payment of import duties. This will involve hiring a freight forwarder or customs broker, both of whom will work out how much duty you need to pay and will make sure all the appropriate documentation is filed. You can find lists of freight forwarders/customs brokers in your area by doing a Google search.
e.g. '[california:9eff8a08] + freight forwarder'.
For smaller imports you may prefer to use a shipping company such as DHL or UPS. These services will include duties in the overall shipping price.
Check out this article, you might find it useful,
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Also, you might want to search the forum for DHGate related posts to see what other members have to say about them. Just click on the 'Search Forum' link from the navigation panel on the left-hand side of the page, type DHGate in the 'Search for keywords:' box, then hit Enter.
All the best!