I guess I'll see if it gets returned. I've received lots of stamp purchases mailed with the '54 20c & '57 30c issues. One piece was sent with tracking and my mailman brought it to the door and was amazed to see the old stamps on the cover. Never said a word about being invalid.
Agreed, this kind of thing can make it through the mail stream. During the vast majority of the mail stream, an individual piece sees very few human eyeballs.
Mailing promotional items or other mail that is not important is probably worth the gamble (assuming a person does not have an ethical issue with intentionally using invalid postage).