First of all, I think the Post Office does an excellent job – where else can you pay someone less than 50 cents to take a letter to California from Boston?
Second, why doesn’t junk mail pay full price to have someone deliver their mail? It would solve 2 problems: Get rid of unwanted junk mail and add money to the Post Office.
Third, there are some services that aren’t meant to make money, we pay taxes to provide services: example, trains and commuter rail, the post office, the IRS, etc.