Applying A Ww Stamp To A USA Mailing

Are there any postal regulations that prohibit applying a non-US stamp to outgoing mail as long as it has the proper US postage?
IE: I have thought a number of times about putting some kind of topical WW stamps on mail going to one of my grand kids.
Thanks for your responses.
Paul

Some stamp collectors like a bit of hijinx.
I have on occasions received mixed stamps on my covers from US friends.
...and all sorts of cinderellas on the back.

Two that come to mind.

Monaco.


Wallmart "Flag over Porch"

I don't have any examples but I do it all the time. I use a lot of those old low value foreign stamps with gum to add to the front of the envelopes and also uses seals for the back flap.

Or using a foreign stamp to pay part of US postage! Both of these were mailed during the 29 cent rate period - with the second cover "generously" overpaying the postage by 1 cent!





In both cases, modern processing technology makes this very easy. The tagging on both the low value US stamps (still being done then) and on the foreign stamps enabled the letter to get to a bundle for the business destination without much more than a glance or two at the address by a human.

I find the examples in this thread rather ironic. I have a friend who in 2007 used one of the new Jamestown triangles on a piece of mail. It was returned to him undelivered with the admonition to please use U.S. postage. Presumably a postal clerk simply noticed the shape of the stamp & assumed it was foreign postage without carefully examining it. I will have to tell my friend that, had he used a "normal-appearing" foreign stamp of appropriate denomination, he might have had better success.