CBP makes the final determination of what the correct rate of duty is, not the importer. The Harmonized Tariff System (HTS) provides duty rates for virtually every item that exists. The HTS is a reference manual that is the size of an unabridged dictionary.
Experts spend years learning how to properly classify an item in order to determine its correct duty rate. For instance, you might want to know the rate of duty of a wool suit. A classification specialist will need to know, does it have darts? Did the wool come from Israel or another country that qualifies for duty-free treatment for certain of its products? Where was the suit assembled, does it have any synthetic fibers in the lining?
The U.S. International Trade Commission-Tariff Database, is an interactive data base that will enable you to get an approximate idea of the duty rate for a particular product. Please be aware that the duty rate you request is only as good as the information you provide. The actual duty rate of the item you import may not be what you think it should be as a result of your research.
CBP makes the final determination of what the correct rate of duty is, not the importer. For very specific duty information on a particular item you may request a Binding Ruling. You may also receive guidance by calling your local CBP port.
Related: How to determine Vietnam’s import duties (to be provided)