Step 1: Getting the right materials; for single cards it is best practice to use a penny sleeve and a semi rigid holder.
Note: The semi-rigid holder allows for "no" movement.
Step 2: Insert the card into the penny sleeve.
Step 3: Insert the penny sleeve with the card into the semi rigid holder.
Step 4: Ship the card using a bubble mailer and put in between two pieces of cardboard, for that added protection.
Alternate shipping: would be to use a penny sleeve, a top loader , and to tape the top of the top loader, which "may" lead to movement of the card during shipping.