I'm not sure if "envelope origami" really exists but for my amusement lets pretend it does.
Envelope origami - to create a likeness of an object using geometric folds and crease patterns preferably without gluing or cutting the paper, and using only one envelope.
In this case, I folded an envelope into a box. It's an easy craft to turn your junk mail into something functional. Security envelopes work best for this project.