Couriers take tens of billions of dollars across the border to Mexico every year, stashing the cash in spare tires and baby diaper shipments, The Washington Post reports. Customs agents on both sides of the border are overwhelmed by the task of stopping all that money—they seize about one percent of the cash—though officials say that job is essential if violent drug gangs are to be stopped. Mexican cartels simply write off the tiny amount of money they lose at the border as the cost of doing business, because they lose a smaller percentage of their revenue than one would pay in service fees for withdrawing cash from an ATM. Cash is transported in bricks of $20 bills wrapped in plastic and often left with the sticky residue or powder from marijuana, cocaine and methamphetamine. Customs agents have found record amounts of cash since Mexico and the U.S. began joint operations in March 2009, but they’re still hardly making a dent in the drug gangs’ income.