Originally Posted by Favorite Fabrics
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!
Definition of Priority Flat rate - If it fits it ships. There are assorted sizes from envelopes to parcels provided free from USPS. (If you over stuff, the contents are at risk.)
Definition of Priority Lg envelope - 12" high X 15" wide and no thicker than 3/4". your own envelope, not rigid and even thichness, no lumps. Next step is a parcel.(What a deal with the FRE.)
Side flaps on Priority FRE will give if they are exposed to moisture. This will happen with wet weather. It is impossible to shield hampers from rain when moving and not all facilities have covered docks. If the envelope is buldging I recommend plastic envelope/zip bag and tape to reinforce. This link may help - (Retired from the PO)
http://www.usps.com/prices/first-class-mail-prices.htm