What is the proper procedures and guidelines to mail a shotgun?

When you are shipping a gun use UPS. USPS will not accept it and I'm not sure about FedEX. They will give you the least amount of trouble. As for shipping it wouldbe like shipping any other item. But since it is going over state lines you have to ship it to an FFL for transfer. Even to immediate family as it is crossing state lines and therefore the interstate commerce clause. The only exceptions to that is if you are shipping a gun to the factory for repairs and from them back to you as you own it or to yourself if you were say moving out of state.