WASHINGTON — The U.S. Postal Service finished fiscal 1987 with a $223-million loss, the agency reported today. Total revenue for the year which ended Sept. 30 was $32.5 billion, while spending was $32.7 billion, according to Crocker Nevin of the agency's governing board. That contrasted with 1986, when the independent agency was $305 million in the black. In 1985, the agency lost $251 million.