Models of this type are also automatically listed in: geometric, origami-first design, single-sheet

A number of envelopes, both plain and decorated, can be folded in the form of origami without the need to use cuts or glue.

This page lists models of a single type. You might be interested in folding instructions instead.
Postal Envelope

Postal Envelope

An origami envelope which looks almost like a classical glued envelope, but can be closed by folding only. If you use it for sending a letter by actual posta...