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.
This may be somewhat off topic but part of this might be useful.
Some years ago I decided to send out antique Victorian Christmas postcards to close family and friends. They were all in unused condition and were all well over 100 years old but none were particularly rare. They were valued mostly around $10 which I thought wasn't a lot. I know people who spend nearly that much per person on fancy Christmas Cards that go in envelopes. But here is the twist. I found old unused postage stamps from the years the postcards were printed or in some cases approximate years. They were of course in the correct a amount for post cards but just all stamps that were issued 110 to 120 years ago. Yes I paid a lot more for them than I would for stamps issued this year but none were high end stamps. None had Scott values over $10 and most had minor defects that would not be noticed by the recipient as none were stamp collectors but I thought they would be a nice touch. For a bonus our post office at the time was using special Christmas cancellations in an attractive Victorian style. Since I belonged to the stamp club that met weekly at that post office where our philatelic clerk was a member (and vice president) I had heads up advance notice of that special cancellation which gave me the idea for sending out the Victorian Christmas postcards with the stamps from the period.
My point is you can often find an appropriate vintage stamp on eBay or elsewhere that fits any theme including gag or joke gifts or practical jokes.
I have also sent out antique Christmas postcards in past years. When I found unaddressed ones in 25¢ boxes I'd buy them and would usually have a couple dozen at year's end. I didn't go to the trouble of finding period postage, but my friends still appreciated getting old and unusual cards.
I think mailing antique cards is a fine idea and might even generate new collectors. Indeed, they will not be missed -- but you might avoid mailing cards with Zeppelins and Queen Victoria RPPCs.
The budget hotel where I stayed in Tokyo in 2010 and 2015 (now closed, alas) had postcards from former guests on the walls of the lobby, so I mailed them one with this street scene from my neighborhood.