A 23-year-old Medway man has been arrested after a package containing approximately one pound of the club drug known as “Molly” was seized at Logan International Airport, according to a State Police statement on Sunday. The package of MDMA, an illegal stimulant that is a form of ecstasy, originated in China and was intercepted by authorities at Logan Wednesday, State Police said. A trooper posing as a FedEx driver took the package to its delivery address in Milford, police said. The recipient identified the owner, who was arrested when he came to pick it up, police said. David Sheehan of Medway was charged with possession of a class C substance and conspiracy to violate drug laws.