Can the "stamps" on pre-printed postcards be cut off and used as valid postage?

When we got married, we bought a ton of pre-stamped post cards (like this, but a different design) to use as RSVP cards in the invitations. We have approximately half a million of them left unused, gathering dust. If I were to snip off the “stamp” (which is printed-on) and affix it to the corner of a letter or package (in sufficient quantity as to cover current first-class postage), would this be considered legal postage, such that my parcels would reach their destinations?

It’s such a waste to throw these out, since we spent $0.24 on each one – but I never send postcards. I would like to be able to rescue them for practical use.