Import fees, called duty, are assessed when crossing the border with purchased goods.
According to export.gov, a duty is "a tax levied by governments on the value of imported products. Sales and state taxes, and in some instances customs fees, will often be levied as well." The terms "duty" and "tariff" are interchangeable.
The duty is assessed when the goods are imported, along with any other fees or taxes that may apply.
Assessing Duty
When bringing items through customs into the United States, your duty will be calculated at that time. You will have a personal exemption that will result in some of your purchases being duty and tax free. The exact amount of this exemption will vary depending on what country your are bringing the items from and the total declared value of the items. The exemption can only be used once every 30 days.
After your exemptions are subtracted, a flat rate will be charged on the next $1,000 of merchandise being imported. If the dutiable items total more than $1,000, then the remaining amounts will be assessed at the current duty rates.
Special rates will apply to items that are made and purchased in Canada or Mexico.
Tobacco
It is possible to import tobacco products. However, you may only import them in quantities that are within your exemption limits. Any tobacco products that are presented without a valid exemption will be seized and destroyed at customs. An example provided by CBP.gov explains that when returning the the United States, you may have an $800 exemption that includes not more than 200 cigarettes and 100 cigars. If you declare 400 cigarettes, then you would be allowed to bring 200 cigarettes, tax-free, under the exemption. The remaining 200 would be destroyed.
The tobacco exemption applies to all adults. However, no traveler may import Cuban-made goods. This includes Cuban cigars.
Alcohol
You may import up to one liter (33.8 fluid ounces) under your exemption if the following apply: You are 21 years of age or older, you will be using it for your own benefit or as a gift and it does not violate the laws of the state that you arrive in.
You may bring more than one liter if it is strictly for personal use, but you would be required to pay duty fees and taxes on the additional amount.
While there is no limit on the amount you may acquire for personal use, if you attempt to import an unusually high quantity, this could incur suspicions that you intend to resell it. According to CBP.gov, "CBP officers are authorized by the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) to make on-the-spot determinations that an importation is for commercial purposes, and may require you to obtain a permit to import the alcohol before releasing it to you. If you intend to bring back a substantial quantity of alcohol for your personal use, you should contact the port through which you will be re-entering the country, and make prior arrangements for entering the alcohol into the United States."
