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 Jargon File version 4.3.1, ed. ESR, autonoded by rescdsk.