Two attempts.

The first time I ripped off the end of the envelope and I left a small cardboard piece in it with a note talking about what I've enclosed. Of course, I didn't enclose anything else. I figured it'd fall out and the USPS would determine it was missing parts and I'd get my bag. I was wrong. They delivered the envelope with the piece of cardboard in it normally.

So I decided to take an envelope and reduce it to the minimum possible pieces: Area for a stamp, the address, and a flap at the bottom so it's recognizable as an envelope.

Voila!