I like these instructions of the levergun.com site:
"Packaging must be of sufficient quality and strength to tolerate necessary handling and inevitable mishandling; you must seal box sufficiently to keep it closed – use plenty of plastic "boxing" tape. I recommend the following approach, see note at end of this section:
If necessary, purchase a lightweight and affordable hard gun case, such as the Gun Guard, or similar type (you need such a single-gun case and a cardboard container in which that single, single-gun case was shipped);
Securely tape an address label (with your "From" address and the above noted "To" address) to gun;
Insert additional foam rubber or bubble-wrap into case, as necessary, so that gun is centered within case and is positively prevented from moving around inside case – with sufficient added foam rubber or bubble-wrap, case will be rather difficult to close and latch;
Securely tape an address label (with your "From" address and the above noted "To" address included) to outside of gun case;
Insert cased gun into box in which gun case was shipped, add Styrofoam "peanuts" or other similar material to prevent gun case from moving inside cardboard box (as necessary, completely mark out all exterior markings on box, using a black permanent marker);
Leave end of box open.
Take boxing tape and a "Sharpie" style permanent marker with you to Post Office;
Either label a clear area of largest side of box with your proper return "From" address and the above noted shipping "To" address (preferable method) or securely tape an address label to correct area of outside of gun case box (run tape wraps fully around box, as necessary, to completely cover address label);
After purchasing proper postage stamp for entire package (which will include cased gun, shipping box and a manila envelope containing completed order from and a Postal Money Order), apply postage to upper right-hand corner of addressed side of box;
Obtain Postal Money Order in correct total amount to cover costs of all services ordered plus cost of return shipping and insurance; I suggest a minimum of $750 insurance (return shipping and insurance will be significantly greater than cost of shipping & insurance for shipping to Shooters World, you can get this amount by adding the value of the money order to $750 and asking the postal worker for the cost of that much insurance and shipping on the same package);
Insert completed Money Order with your completed order form into a manila envelope, seal this envelope and write M.L. McPherson on outside;
Insert this sealed envelope into open end of box;
Securely seal box using boxing tape;
Apply several layers of tape over each end of box in each direction, run tape ends about 4 inches down sides of box end;
Apply tape wraps around box, near each end (to lock tape across ends of box);
Apply tape wraps at center and 1/4th points around length of box;
Apply tape wraps in any other area of potential weakness;
Mark box "fragile" and post.
If you cannot ship the gun packaged as described above, include an additional $50 to total costs to cover my time and costs required to properly package it for safe return posting."