I'm SWOP and I'm a super Sustainable Way of Posting because I can be reused many times for delivering parcels. I'm also made from totally natural, renewable materials - FSC paper, organic cotton, soy-based inks and a plant-derived coating. I come in two versions; one with a velcro strip and one without. Our testing has shown the velcro isn't required to seal these bags (folding and sealing with a label is sufficient), however if you are sending particularly heavy or precious items, you may want the added security provided by the velcro strip.