Is a rubber rat a toy or a practical joke? For businesses that import articles into the United States, this kind of hairsplitting can be worth money. Import duties on toys, for example, can be almost 50% higher than duties on practical-joke items.

When customs officials recently slapped the "toy" classification on a fake rat imported by a $30-million New York City-based wholesaler of novelty items, the company consulted a customs lawyer. According to the arcane logic of the U.S. Customs Service, joke rats have tails of six inches or more, whereas toy rats have tails under six inches. To avoid the higher levy, the company made sure that future shipments of rodents had longer tails.

"A company should prepare for customs laws the way it prepares for taxes," says Irving Mandel, founding partner of Mandel & Grunfeld in New York City, the country's largest law firm specializing in trade and customs matters. "Companies routinely consult tax experts to deal with the Internal Revenue Service, but instead of consulting customs-law experts to find flexible parameters within customs law, they just blindly pay import duties as if they were fixed costs. A business can sometimes end up paying higher tariffs than a competitor who imports a similar item."