The industry's standard for rigid mailing envelopes. • Made from .036 chipboard, 25% stiffer than all other paperboard mailers. • 100% Recycled paperboard features a 97% Post-Consumer Recycled Content. • Featuring a reusable, economical locking tab closure. • Rigid construction - will not bend or crush. No additional stiffeners needed. • Practical alternative to bubble packs or shredded paper padded mailers. • The First Dimension for each mailer listed below is the size of the opening.