A document marked 'Do not bend' still needs packaging that can protect it during normal handling. The words describe your preference; they do not make a thin envelope rigid. Choose the mailer according to the document's size, replaceability and tolerance for a crease.

A certificate, signed original and ordinary photocopy may all fit the same envelope while needing different levels of protection. Start with the finished item, including any sleeve or backing. Measure that assembly and allow enough space to insert and remove it without catching corners. Excess room is not always helpful if the document can slide around inside.

A board-backed envelope may suit some paper items, but check which face is supported and how the closure behaves when loaded. A fully rigid mailer offers a different kind of enclosure; it can also be treated differently by the carrier. Protective sleeves may help with scuffing or incidental moisture, but do not assume any package is waterproof unless its specifications say so.

Before buying a box of mailers, try one complete example:

  • Insert the document with its intended protection and close the mailer normally.
  • Check the corners and edges for pressure points or excess movement.
  • Measure and weigh the finished package, not the empty envelope.
  • Confirm the carrier's category and service for its dimensions, flexibility and thickness.

USPS distinguishes flexible large envelopes from rigid or otherwise nonqualifying pieces. Adding stiff board can change the postal classification, so a mailer advertised for documents is not a promise of letter or large-envelope postage. Use the current guidance for the finished item or take a sample to the counter when the category is uncertain.

Place the address where it remains clear and flat. Avoid a label over a fold or closure that will be disturbed during handling. If the document has lasting value, think about how the recipient will open it too: a tightly fitted original immediately behind the cutting edge of an envelope leaves little margin for error.

Keep a note of the mailer dimensions, the document size it protects and the service used. For repeated mailings, a retained sample makes later supply changes easier to check. When the contents change, revisit the package rather than assuming the old postage and protection still apply.