WASHINGTON (Reuters) - The financially beleaguered Postal Service suffered a setback in its plan to end Saturday delivery of first-class mail as Congress on Thursday passed legislation requiring six-day delivery. The Postal Service, which lost $16 billion last year, had announced last month its plan to switch to five-day mail service to save $2 billion annually. No law requires the Postal Service to deliver mail six days a week, but Congress has traditionally included a provision in legislation to fund the federal government each year that has prevented the Postal Service from reducing delivery service. The House of Representatives on...