Do you go through customs coming back from Cancun?
When flying from Mexico to the United States, passengers will go through customs and immigration checks at their first point of entry into the US. This means that the airport where you first arrive in the US will be where you go through customs.
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If you enter or leave Mexico, you must declare any sum of money equaling US$10,000 or more that you bring in cash, checks, money orders, other cashable documents or a combination of them. Failing to declare carries a fine ranging from 20% to 40% of the exceeding amount.
Travelers must have a valid and current passport or a passport card to enter the country. This passport will be stamped. You will need to show this stamp when you depart from the airport. “Passport Cards” can only be used for land border crossing and are not permitted for international air travel into the country.
If you're planning on traveling by land or sea, usually a passport card will suffice, though it's recommended to have a book in the event of an emergency flight. For some closed-loop cruises, i.e. originating in and returning to the US on the same trip, a state-issued driver's license can be enough.