Rigid Card Envelopes
These are ideal protective mailers that can be used to protect books, brochures, calendars, etc.
They are often called Capacity Book Mailers, or Card Mailers,
PIP - All rigid card mailers up to 249mm x 352mm are sized to fit Royal Mail's Large Letter size limits (Pricing In Proportion) and can be used to save costs compared to traditional boxes. They are also lighter-weight compared to boxes.
They are also handy for loose items that would otherwise bunch up in mailing bags, to ensure they will fit within Royal Mail's large letter size limits.
These are made from strong 400gsm card in natural manilla brown.
Amazon often use Rigid Card Mailers like these for sending books and other small/thin items that need extra protection that normal envelopes don't provide, and as are designed so they fit through most letterboxes.
They have a peel & seal closure that are easy to close by the seller, and the receiver has a handy rip & strip open, making it easy to open the package.