I am intrigued by the law-abiding direction the responses have taken. I'll bet if the request had been "Post your cinderellas which passed free through the mail" we would have several dozen images posted by now! Yes, they are collectible and I have bought several from $1 boxes.
' You can create lots of cool covers without cheating the post office.
Folks who collect charity seals put a great premium on covers where the seal was placed near the postage stamp so that both 'caught' the postmark. (The result is a seal 'tied' to the cover.) Nothing materially wrong with doing this with any cinderella.
Immediately after the attack on Pearl Harbor - but before Germany declared war on the US - Herman 'Pat' Herst sent five covers to fictitious persons at largely fictitious addresses in Germany. He was hoping that the returned covers might have wartime embargo markings and, if memory serves, he was two for five.
Yes, that imposed some unrecompensed costs on the ReichPost. Yawn.
You can also invest in a Mailer's Postmark Permit. This enables you to not only festoon covers with postage, but to add a legible postmark.
I think that the occasional well-adorned 'philatelic' cover adds a nice touch to any collection; after all, it says that somebody cared.