Coins are the easiest thing on this site to pack well and the most expensive thing to lose.
The method
Do not let it move. A loose coin in a parcel abrades against whatever it touches, and abrasion is damage.
Flip or capsule, taped closed, then card either side, then a rigid mailer.
Do not use PVC-containing plastics. They degrade and damage coin surfaces over time. Inert flips exist and cost almost nothing.
Do not post a coin in a padded envelope alone. Sorting machines are unkind and the coin's outline is visible through the padding, which is its own problem.
For notes
Rigid, flat, and never folded along a new line. A fold that was not there before is permanent damage. Sleeve, card either side, rigid outer.
Security
This is the category where a parcel's shape tells somebody what is inside. Keep it flat, keep it unremarkable, and do not write anything on the outside that describes the contents.
For high value, use a service with real tracking and a claims process, and discuss it with the buyer first.
Humidity
Metal spots in damp. Silica gel in the parcel costs nothing. Monsoon and your stock for the storage version.
Questions people ask
Can I post coins by ordinary post?
For low value, yes. For anything serious, use something tracked with a claims process.
Should I insure it?
Weigh the cost against the value, and remember the buyer is covered either way — the claim question is between you and the courier.
What if it arrives damaged?
Photographs of the packing settle it. Take them.
More: packing an order.