1. They have been bailed out numerous times. Yes, they paid their debt back but it was lent at a preferred rate by congress to an entity that no commercial lender to would do at those rates. (see Postal Service Reform Act (H.R. 5714))

2. The Post Office is except from all local state, city, and federal taxes. That also includes road tolls, vehicle registration, etc (wouldn't Fedex love that)

3. The Post Office has an open line of credit from Treasury under preferred rates and terms that no regular business would get.

4.Finally, they hold a monopoly status on delivery of first-class mail. This monopoly status, and the legal issues surrounding "whats considered legal notification" is tantamount to an indirect tax.