Even though, as you say, there is no other reference to the Post Office etc in the text, once we do see the wider context of this passage the meaning of the sentence you supplied becomes clear.
Here's what I found:
"If you are part of the group which you are addressing, you will be in a position to know the experiences and problems which are common to all of you and it'll be appropriate for you to make a passing remark about the inedible canteen food or the chairman’s notorious bad taste in ties. With other audiences you mustn't attempt to cut in with humor as they will resent an outsider making disparaging remarks about their canteen or their chairman. You will be on safer ground if you stick to scapegoats like the Post Office or the telephone system."
It appears to be advice on how to include humour when giving a talk or lecture. What it's saying is that when you are addressing a group that you are not a part of, it's risky to make fun of that group's own institution or office and safer to make fun of more remote targets, like the Post Office or telephone system. We've probably all had frustrating experiences with such corporations and they are a target for humour that everyone can identify with.
not a teacher
Student or Learner