Cards are cheap to pack properly and expensive to pack badly.
The method
Sleeve the card — a penny sleeve, card going in the right way up.
Toploader or a semi-rigid holder over the sleeve.
Tape the toploader closed so the card cannot slide out inside the parcel. Tape the holder, never the card.
Rigid outer — a stiffened mailer, not a soft envelope. A soft envelope goes through a sorting machine and so does the card inside it.
Team bag or cling film around the toploader if there is any chance of damp.
For a graded card
The holder is rigid but it is also brittle at the corners. Bubble wrap it and keep it from moving — a cracked holder means re-grading, which costs money and a fortnight.
For a lot
Bundle in stacks with card between, not loose. Loose cards in a box abrade each other and arrive worse than they left.
Heat and damp
The two slow killers. A parcel sitting in a van in May is hot enough to warp, and humidity gets into a sleeve and stays there. Bag against damp and do not post into a known heatwave if you can wait a day.
Monsoon and your stock covers the storage side of the same problem.
Questions people ask
Is a toploader enough on its own?
No. It needs a rigid outer or it bends with the envelope.
Should I write "do not bend"?
It costs nothing and influences nothing. Pack as though nobody reads it.
What about very valuable cards?
Double-box and photograph the packing.
More: packing an order.