Looks like China Post has their own understanding about their own rules in different areas.
Today I went to the post office to send registered mail. And in Beijing, it's fairly smooth. Didn't encounter any problems. But from what I read from other posts, in some places they will ask you to open the letter so they can check what in it. They shouldn't read what you write on the paper, but they will check what you have other than paper. Technically speaking, the law gives the officer the right to check the content, but in my humble opinion, if you can't sneak a bomb into a flat 11cm by 22cm envelope, then you shouldn't bother others. Most of the time, they just bother you not because of you, but simply because they can. And that's probably why I choose to stick with Beijing.
In Beijing, they won't bother you not solely because they are better people, but also because I can ride another 20 minutes and go to the HQ of China Post and send a complaint if they bothered me.
The power of supervision :)
I asked the teller (I should use teller instead of officer. I naively though the people works in the office will be officer) if she wants to check the content, she said there is no need to do that. I can just seal it.
So I pull out my stamp and the paste. I saw a flash of shock on her face when I stamped the seal.
Oh, just find out the word "stamp" can be postage stamp, or the one I used to print a pattern/mark on paper. No wonder why I feel similar to that word.