The U.S. Postal Service, an independent agency that receives no taxpayer money for operation, is a 200-year-old institution still tethered to Congress by centuries-old shackles.Partly because of that impediment, and changing communication habits in a technologically advanced society, the Postal Service has been losing billions of dollars the past few years -- $15.9 billion in the last fiscal year alone. This has happened even though it reduced staff, combined operations and cut hours in some underused post offices.After months of imploring Congress -- with little success -- to approve cost-cutting measures, including closing thousands of postal stations, Postmaster General Patrick Donahoe announced Wednesday that the USPS will stop Saturday letter delivery. Package deliveries, an area where the Postal Service has increased business, would continue six days a week, as would service to post office boxes.The move is expected to save $2 billion annually. But it also conflicts with a 30-year mandate from Congress: Appropriations bills routinely call for six-day delivery.Donahoe says he has authority to move forward on the change, which would take effect in August, because Congress hasn't approved an appropriations bill and the government is operating under a continuing resolution (a temporary spending rule). That temporary measure expires March 27, so lawmakers still would have time to stop the plan if they want to.Some members of Congress have expressed support for the proposal, but others argue that cutting Saturday service would hurt rural areas and some businesses.For the Postal Service to return to stability, much less profitability, it must continue to modernize and improve its ability to compete.The USPS has 546,000 career employees and operates the world's largest fleet of civilian vehicles, according to the agency website, about.usps.com.While many in Congress understand this, they've found it difficult to consider changes to an institution that their constituents often see as not just a vital service but also a local landmark or an important community center.But if the agency is to remain any of those things, Congress must let it be a viable business.