[aamar]

Well, on the one hand, a legally-protected monopoly is not the same thing as mandatory. Anyway, UPS/Fedex/DHL may be operating under a loophole, but the loophole exists: it's easy to send a sheet of paper through services like those.

The kind of regulatory problems ("classified differently") you describe are unfortunate, but those affect all major industries and are not specifically relevant to whether using the postal service is being protected. In other words, either UPS or FedEx having regulatory advantages doesn't mean that the overall tax/regulation regime is constructed so as to give the Postal Service a market advantage.

edit: see protomyth's comment; the Postal Service is unionized, so if anything FedEx seems to have the market advantage here.