I get so annoyed when I have to make a special trip for mailing supplies - especially when I'm just sending something small! I've modified my first recycled envelopes to be sturdy and mailable by using a cereal box as the raw material.
I used a greeting card envelope as a template and traced it onto my dissected box - put it right up to the box's opening flap, this way you create a natural flap for your envelope. Trace the envelope rectangle again underneath the one you just did. Then, add two side flaps to your first rectangle.
When you cut out your envelope it should look like this:
Next, I scored along the division between the two rectangles to make a clean fold. Last, fold up your envelope and use clear packing tape to seal the edges.