Packages that do not pass free of duty will have a yellow Customs and Border Protection (CBP) Mail Entry form (CBP 3419A), attached to the outer wrapper. A CBP Officer will have already filled out this form with the item's tariff classification number, rate of duty, processing fee, and the total amount that must be paid in order to take possession of the shipment.
You should keep this yellow form until you are certain that you intend to keep the merchandise.
CBP then sends the parcel back to the Postal Service, which sends it to a local post office for delivery.
Note: Most personal shipments worth up to $200, and gift packages worth up to $100, will pass duty-free as long as the recipient does not receive multiple packages in a single day whose cumulative value is more than these amounts.