In my day we didn't have all these fancy stock sites to go to to fulfill all of our creative needs. We had to make things by hand (mouse?), on a computer, like suckers. Anyway here's an old envelope icon I made. Feel free to use in your work, share any examples that you do :)
FREE download here:
https://www.mediafire.com/file/63sqjya0y7wttav/Envelope_Icon.eps/file