The word "stamp" is confusing here.

You put postage on the letter, and the post office stamps the postage (to invalidate the postage). The stamp contains a date. You can't stamp something after having mailed it, that happens as part of the mail submission.

OK, I suppose this could be arranged. The seal has to be over the whole of the "back side" of the envelope, where the flap is. Then put the address and the postage over that, and the post office stamps it.

What confused me was how you would achieve post office stamping over a seal that's on the wrong side of the envelope, where the flap is.