Source : Free On-Line Dictionary of Computing

postcardware
     
        {Shareware} that borders on {freeware}, in that the author
        requests only that satisfied users send a postcard of their
        home town or something.  (This practice, silly as it might
        seem, serves to remind users that they are otherwise getting
        something for nothing, and may also be psychologically related
        to real estate "sales" in which $1 changes hands just to keep
        the transaction from being a gift.)
     
        [{Jargon File}]