A lady at the post office taught me this trick and I have realized very few people know about it! You can turn a flat rate envelope into a pouch to increase the capacity. It holds more than a small flat rate box ($5.80) and about as much as a medium ($12.35) for only $5.60. So worth it! I do this all the time! Just fold the bottom both ways, and flatten it out from the inside, folding the corners and taping them up into a rectangular bottom. Secure with lots of tape. Happy shipping!