This envelope is not a rigid mailer. The envelope is designed to make soft or flat products closed Tight in the envelope. So you can meet under 20mm thickness target easier, while other soft normal paper envelopes or poly mailing bags will just 'inflate' with a soft item inside. If you are after a rigid hard mailer, this is not one. If you need something to send you flat or soft products, this is an ideal one for cost and postage saving.