answer: #Person1# wants to send a letter by ordinary airmail. Since it contains a postal order, #Person2# recommends #Person1# to have it registered.
documents: #Person1#: Would you please weigh this letter to see what the postage is?
  #Person2#: Do you want to send it by ordinary or registered mail?
  #Person1#: By ordinary air mail, please.
  #Person2#: Anything of value in it?
  #Person1#: A postal order for four hundred dollars.
  #Person2#: In that case, you'd better have it registered.
  #Person1#: Will I be informed when my friend gets the letter?
  #Person2#: Yes, when your friend gets it, he'll sign a receipt, which will be sent to you by mail. Then you can be sure it's been received.
  #Person1#: All right, I'll have it registered, then.