The Jargon File (version 4.4.7, 29 Dec 2003):

postcardware
 n.

    A kind of 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.)


The Free On-line Dictionary of Computing (30 December 2018):

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]