I checked and see that to send from Australia to Peru it costs Aus$3, or around S./7.30, so that is a pretty big difference with S./11.80 in the other direction. My best bet is that it is a question of critical mass of letters that the mail service sends here. Not very many people seem to use the postal system, so the cost per send works out to be much higher.
Also, state run enterprises are not very efficient here either, so that probably plays in, too. It turns into a vicious circle. Anyway, pure speculation, but both reasons seem to make sense.
What surprised me a little bit last time I had to mail internationally was how Serpost also asks for picture ID in order to accept the letter. I suppose that is because of the fear that drugs or fake bills will be mailed.
Great question, btw!