You could tack it shut just enough to get by, but not actually full seal it. For that matter, you could probably just stuff the outer flap down inside the envel

You could tack it shut just enough to get by, but not actually full seal it. For that matter, you could probably just stuff the outer flap down inside the envelope and maybe put a very small piece of tape on it. Same difference in the end. The point being, mailing yourself an envelope doesn't really prove anything about the provenance of whatever is found inside later.