The United States and Cuba have agreed to re-establish direct postal service, officials of both countries said Friday, the latest progress in President Obama’s push to normalize relations with the island nation after more than a half-century of estrangement. The process will begin within weeks with the launch of a pilot program, which will facilitate mail flights between the two countries several times a week. Mail between the United States and Cuba currently must be routed through a third country, in line with the American embargo and a complex web of Treasury and Commerce Department sanctions that tightly restrict what kinds of materials may be sent, to whom, and under what circumstances. American and Cuban officials struck the agreement to begin allowing direct service on Thursday in Miami.