Customs officers operate at the San Ysidro border crossing, at San Diego International Airport, and in the bay at Shelter Island.
You're always allowed to bring goods of a certain value back home without having to pay any duty or import tax. But there's a limit on the amount of tobacco and liquor you can bring back duty-free. If the total value of your goods is more than the duty-free limit, you'll have to pay a tax (most often a flat percentage) on the value of everything beyond that limit.
U.S. Customs and Border Protection (www.cbp.gov.)