While U.S. citizens can take an unlimited amount of money into Mexico—you will have to fill out an IRS form at U.S. Customs if it’s more than $10,000—you’re allowed to bring back only $400 worth of merchandise every thirty days duty free. (If there are four people in the car, you can pool your exemptions for a total of $1,600.) I asked Pat Coss, a public affairs officer with the U.S. Customs Service in Washington, how an agent can tell if someone crossed with the limit the week before, or even the day before, at another bridge. There was a long pause on the phone—for a minute I thought she had hung up—before she told me how customs agents are highly trained to detect subtle signs of deception. Right. I think it’s simply not their highest priority. The agents do, however, have a firm grasp of market values. If you exceed the limit, the duty this year stands at 7 percent, thanks to NAFTA. The original 10 percent duty has been dropping one percent a year since the trade agreement was enacted, in 1994. By 2004, the duty will have been phased out completely.