You can send a banana..
Step One: Put a stamp on one end of the fruit
Step Two: Repeat step one, but put the second stamp on the other end
Step Three: With unwashable marker/pen, write on the return address and address.
Step Four: Put in mail box.
Step Five: Wait.
Step Six: Get a hysterical letter from your friend in the mail.
(Hey, don't sue me if this isn't correct, but my friend's friend (lol)) tried it. It worked..)
TIP: Probably want to use an un-ripe banana; ya don't want ur friend to receive a mushy, brown banana, do ya?!