Abstract: In order to ensure the prompt processing of payments of bills with personal checks while using automatic mail opening equipment, a return envelope assembly includes a front panel and a rear panel each having a top edge, a bottom edge, and a pair of side edges. The front and rear panels are integral along their bottom edges about a bottom fold line about which they may be folded so as to have their inner surfaces lie in confronting relation after the envelope is formed from a one piece blank. A side flap is integral with each of the side edges of the front panel about respective side fold lines which are structured such that the side flaps may be folded inwardly to lie between the confronting inner surfaces of the front and rear panels. The side flaps are secured to the inner surface of the rear panel using an adhesive and a closure flap is provided to seal an opening defined by the top edges of the front and rear panels.