This funny envelope is found in Home Decorating With Origami, Japan Publications (ISBN 4889960597), page 34.
The model in the book is square when closed, I added a few folds to make it a cuter geometric form. Plenty of other variations are possible!
It is folded from one sheet of A4 printer paper. Opening the envelope is really satisfying, it resists then suddently bursts open in your hands!
This is a gif, if you click you’ll see the opening sequence :)