There is a great thread discussing this. From the thread:
1) Anything over $8000 is subject to taxation.
2) If U.S. CBP decides to tax your imported package, and it is a handbag, and it is from most countries such as Japan (but not Singapore, Australia, etc., due to FTA), the taxable rate is 8% of the declared value of the bag.
3) Many CBP agents aren't the brightest, and the declared value is never written down in USD but in the foreign exporting country's currency and they sometimes calculate wrong. This could end up with higher or lower taxable fee perception by the agent. This can be an instance where you pay more/less than expected. In addition, your shipper may declare a lesser value.
4) At the end of the day, CBP may open your package by random inspection and compare it against what is declared to see if it's declared correctly or incorrectly. Their estimations on market value have no rhyme or reason, which is why you can contest your duties paid - this is why you should always take a snapshot of the form you fill out at the post office when paying your duties. This can be another instance where you pay more/less than expected.
5) If your seller ships through a non-state-sponsored service such as UPS, DHL or FedEx, there may be additional brokerage fees added onto the duties, that are imposed on you. This is never pleasant, so beware. The upside is that when they use brokers, usually nobody opens up packages and just go by what is declared by the sender. State-sponsored services (government carriers, such as Canada Post, Royal Mail, EMS, etc.) change hands to USPS control once crossing the border and leaving customs.
https://forum.purseblog.com/threads/custom-duty-import-fees-from-japan-to-usa.451248/page-11