Often these solicitations for things I don't want or need come with return envelopes. These envelopes are the only useful thing that comes with the junk mail.
Glue down the flap and then fold the cut end down about an inch.
This will be you new flap.
Cut off both corners of the flap at an angle.
Trim off one of the flaps.
There you have your new envelope.
You can get 2 seed envelopes from every junk mail envelope
Now fill them up with seeds!
Have a fun day!