answer: #Person1# wants to send a post order for 400 dollars, so #Person2# suggests #Person1# have the letter registered.
documents: #Person1#: Would you please wait this letter to see with the postages?
  #Person2#: Do you want to sent this letter by ordinary or registered mail?
  #Person1#: By ordinary airmail please.
  #Person2#: Anything valuable in it?
  #Person1#: A post order for four hundred dollars.
  #Person2#: In that case, you'd better have it registered.
  #Person1#: Will I be informed that my friend gets the letter?
  #Person2#: Yes. When your friend gets it, he'll sent the receipt which you send it by mail. then you can be sure he's received.
  #Person1#: All right. I have a registered then.