Do not send loose coins through the post. If they sound like coins or feel like coins then you are leaving yourself open to thievery.
Disguise them using simple techniques like placing them between 2 pieces of cardboard and ensure that it is taped down to the inside of the envelope.
(DO NOT USE TAPE DIRECTLY ON ANY FACE OF THE COIN).
Most of the time, If you are sending a single coin, you will be able to use a standard envelope.
You need to ensure that the packing you use is not too thick as it could mean the price difference between a small envelope and a large one.
If you are sending more coins than you can lay flat and evenly in a standard envelope, use a padded jiffy bag and or bubble wrap or anything that will disguise what it is you are sending.
Personally, I prefer to then wrap the entire package in strong tape to avoid any tears or cuts. Check out this page for more details.