U.S. Customs officials returned a jumbo jet to Eastern Airlines Inc. after the airline filed a letter of credit for $890,000 to cover a federal fine for transporting illicit drugs. Customs agents seized the Eastern L-1011 in Miami Monday when it returned from a flight to New York. The 56 pounds of cocaine was found among cargo carried from Colombia to Miami last Sunday. Customs inspectors said the cocaine was in a mailbag that arrived on Flight 982 from Bogota. The mailbag bore an Eastern baggage tag but was not claimed after arrival, officials said. Customs officials said that, in the last two months, Eastern aircraft have been the target of four drug seizures totaling 494 pounds. Karen Ceremsak, a spokeswoman for the airline in Miami, said Eastern will contest the $890,000 fine.