A comic in a soft envelope arrives bent, because the comic was the only rigid thing in the parcel.
The method
Bag and board each comic — the board goes behind, the open end of the bag taped shut.
Sandwich between two pieces of stiff card, larger than the comic, taped together so it cannot slide.
Rigid outer. A proper comic mailer, or a box. Not a padded envelope.
Bag the whole thing against damp before it goes in.
For a run
Stack with card between every few issues and fill the box so nothing moves. Loose issues in a box slide against each other and arrive with new spine damage.
Graded comics
The holder is rigid but brittle at the corners. Bubble wrap and immobilise — a cracked holder means re-grading.
Damp
The thing that actually destroys comics in an Indian summer. Sealed bag, and silica gel for anything valuable or any route through a wet week. Monsoon and your stock.
Do not
Do not use tape on the comic. Ever, for any reason.
Do not roll it. A rolled comic has a permanent curl.
Do not rely on "do not bend" written on the envelope. Pack as though nobody reads it, because nobody does.
Questions people ask
Is a padded envelope ever enough?
For a reading copy you do not mind bending, maybe. Otherwise no.
Should I photograph the packing?
For anything valuable, yes. It settles arguments.
What if the buyer wants it graded on arrival?
Then packing is not optional — a bend in transit costs them the grade.
More: packing an order.