I haven't had a chance to read everyone's responses, but hopefully these tips might be helpful.
For goods mailed to the United States:
1)
Anything over $200 is subject to taxation. The $800 exemption for duties is only for goods being brought into the country by you when traveling (on your person/in your luggage). Mailed in goods qualify for a $200 exemption but anything higher can be taxed, although will not always be. (official ref
here).
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. (official ref
here).
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.
Technically, anything over $200 is supposed to get taxed when coming in through EMS/USPS, but when it doesn't it's because I think CBP is understaffed or #3 calculates wrong. The large boxes being flagged thing is really when it undergoes #4 above.