In the wake of a largely overlooked Supreme Court decision, U.S. petroleum and petrochemical companies, the manufacturers of tobacco products, machine tools, cars and automotive products, as well as grain producers, and food processors could receive export-tax refunds totaling as much as $2 billion. The Court unanimously ruled on Mar. 31 that a harbor-maintenance tax enacted as part of the 1986 Water Resources Act was unconstitutional. The decision affects only goods shipped by sea -- and not computer and telecommunications products exported by air. "We believe you can go back three and one-half years for refunds," says Robert L. Eisen, a partner and chief of the customs and international trade practice at Coudert Brothers, a New York-based law firm. But he says companies need to file claims before May 7 to be eligible for refunds on taxes paid after October 1994.