No no. It has nothing to do with reusing an actual envelope. It is about making envelopes from a scrap op paper thorn out out of a magazine, book, newspaper or catalogue... something you might have just trown into the recycle paper bin.
There is actually a lot of printed publications, that have pages with a full-page photo just screaming to be reused as an envelope. Well at least it is screaming in my little world. The middle envelope is a page from a Martha Steward magazine, and the others are from a supplier catalogue (from work - about to be thrown out) and a clothing catalogue.
100+ years ago, paper was sometimes scarce and very expensive, and at some point, people simply reused an envelope up to 3 times. A few more of my friends have rediscovered the joy of a handwritten note/letter, so I am in constant need of envelopes - and they really enjoy these colorful envelopes.