Once you present your form listing your items to declare, the U.S. Customs agent will place the ones with the highest rate of duty under your exemption. Then, after subtracting your exemptions and the value of any duty-free items, a flat 3 percent rate will be charged on the next $1,000 worth of merchandise (The rate drops to 1.5 percent if coming from insular possessions). Any dollar amount beyond this $1,000 will be charged at the applicable duty rates.
The flat duty rate may only be used for merchandise that is for your own use or for gifts. As with your personal exemption, you may use the flat-rate provision only once every 30 days. Special flat rates of duty apply to items made and acquired in Canada or Mexico.
If you owe duty, you must pay it upon arrival in the U.S. You can pay in cash (no foreign currency), by personal check (drawn on a U.S. bank and made payable to the U.S. Customs and Border Protection), by government check, money order or traveler's check (if it doesn't exceed the duty owed by more than $50). In some locations, you may pay duty with a credit card, but don't count on it -- inquire ahead of time or be prepared to pay by cash or check [source: U.S. Customs and Border Protection].