Cocaine smuggling could more than double in volume this year, while marijuana shipments are expected to decline by almost a third, according to government estimates. The U.S. Customs Service predicts that 275,000 pounds of cocaine will be brought into the country in 1986, up from 130,000 pounds last year, according to Sen. Lloyd Bentsen (D-Tex.), who released the figures. Marijuana smuggling is expected to fall to about 22 million pounds, from 30.6 million pounds in 1985. Customs spokesman Jim Mahan cited the difference in street prices--$135,000 a pound for cocaine against $600 a pound for marijuana--as well as the large domestic marijuana crop as reasons for the projected decline in marijuana smuggling.