请问HN：关税何时征收？具体由谁来征收，如何征收？
我正在尝试了解关税的操作方面。
- 关税是如何计算的？
- 谁来计算这些关税，他们使用什么工具进行计算？
- 计算是在什么时候进行的，谁来支付这些费用？
我想象的过程是：
1. 出口商想要从越南向美国运送货物。
2. 航运公司将货物装载并发送到洛杉矶。
3. 货物到达洛杉矶港。
4. 进口商领取货物（或大宗货物），港口的关税征收员对货物进行检查，根据他们的评估和关税表计算关税，并在货物放行之前向进口商收费。
这样对吗？进口商如何提前确定费用——是否有在线计算器？进口部门如何确保货物的价值不会被低估？
谢谢！
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I'm trying to learn more about the operational side of tariffs.<p>- How d they get calculated?
- Who calculates them and what tools do they use to calculate them?
- When is the calculation done, and who pays for them?<p>What I'm imagining:
1. The Exporter want to ship something to America from Vietnam.
2. The shipping company loads it up and sends it to LA.
3. Item arrives at the Port of LA.
4. The Importer collects the item (or bulk items), and the port's tariff collectors inspect the shipment, calculate the tariff based on their valuation and tariff chart and charge the importer for the goods before they can be released.<p>Is that right? How does the importer determine the cost ahead of time - is there an online calculator? How does the import department ensure that the value of goods isn't being low balled?<p>Thanks!