I don't think there is a bottom flap on the FRE... but there are sealed (small) flaps on the sides.
We've had FREs returned to us from abroad (undeliverable, or unclaimed) and have never seen the side flaps give way, though frequently the corners and bottom edges are abraded away so that the package has some openings.
Keep in mind, though that those packages have gone twice the distance they should have!