I have questions about the Ethics of Gag-Gifting Postcards.
My friend's adult child is off to a city in Europe for three years, along with their spouse & growing children.
I've found some pristine inter-war mass-photo postcards of that city, and would like to send them to my friend so they can send them to their adult child as a parent gag, eg, "I know you're busy, so I got these for you so's you can send them to me".
Leaving aside the question of who might/not find that funny, the question becomes whether/not to extend the joke by pre-addressing the cards and/or affixing that nation's postage, eg, to make them even more ready-to-post.
A) Extend the joke. The world will not miss these cards if they are 'defaced' by addressing & non-contemporaneous postage.
B) Send the cards "as is". The joke is still the joke.
C) Don't gag-gift postcards to non-collectors, as they exit the hobby and are unlikely to return ... even if, in the words of The Lord High Executioner, "they never will be missed".
My daughter and I exchanged picture postcards, roughly on a weekly basis, in the four years she escaped from home to university. My suggestion would be to give your friend vintage postcards of Noo Amsterdam (or other appropriate burg) to post to them, and let the kids take up the game if they want to. If they don't, s/he can still continue, turning up the guilt-o-meter. References to "your silver-haired parent" should be included.
In any case, pre-stamped cards wouldn't work because of the annual changes in rates.