This envelope takes less than a minute to make, requires no glue nor fasteners, can be made from any size paper (as long as it's square), and can easily be closed/reopened.
Step 1: Start With a Square
You'll need to start with a square piece of paper. 8.5 x 8.5 is an easy proportion to get from standard letter paper, and will give you a size roughly similar to a commercial seed packet.
Here, I'm using a template that I created which has fields for writing in the seed name and other helpful information. It also includes a cut line to make the paper square. I've included this template as a PDF, but it's optional. All you really need is a square piece of paper. (If you do use the PDF and you're printing on letter size paper, make sure that it's set to print "Actual Size".)
For tiny seeds that you're trading, or just to get more envelopes from your paper supply, you can start with a small square for an even smaller envelope.
You can also experiment with different colors of paper for even better organization.
Step 2: Fold in Half Diagonally
Step 3: Flip It
If your envelope is printed, flip it printed side down.
Step 4: Fold in a Side
fold in about 2/3 of the way over.