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 post

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.