Why monopolies are bad

Remember - I studied a couple of semester of economics and one of the first things you learn is the usually bad economic outcome of monopolies. Monopolies for products that cannot easily be subsidized tend to produce bad results at high prices. (Read "Principles of Economics" by N. Gregory Mankiw for more detail and fancy charts and formulas).
Well, the Swiss Post is a good example for that. Competition has been allowed for mail heavier than 500 g I believe, so parcels and other services. But the letter post is still a monopoly of "Die Post" - that's probably why they call themselves "The" post. I have been annoyed by their bad service and high prices in the past, but today I got really upset. I had requested e-banking code and access card from UBS and they promised to send that by registered mail by Friday.
Since I have redirected my mail to mother's and also granted the exclusive right to sign for registered mail to my mother and my brother, I thought this was no issue. One of them could open it and pass it on to me. But no, Swiss Post sent it to my apartment and let my subtenant sign off. It's not that I mistrust my subtenant, but I am really pissed off that they hand our confidential bank documents to randoms. And I have not paid little for the service of having my mail redirected and authorizing someone to sign for me.
I called them a few minutes ago and it wasn't my most friendly call ever. They had the guts to tell me that they need the barcode number first. Well, the document is now somewhere on its way to Japan if Raed has sent it already. I told them I expect to be compensated for all this nonsense and for telephone and mail cost. This service is totally crap. You think everything is cared for, but they mess it up. And they CAN do it, because I have no choice as a customer...