My local FFL has long cardboard boxes in which he receives rifles from manufacturers and other individuals. I can pick them up for free when I need them, and keep a supply.
I always remove scopes if I ship a rifle.
I wrap the rifle in a LOT of large bubble wrap as well as other packing material. Nothing moves inside that box when I'm done. Seal it and ship it.
I've had more rifles damaged when shipped in a hard case than when shipped in a well packed box. The plastic (Plano or similar) cases will let your rifle slide around inside the case and do damage. I distinctly recall one Weatherby barrel that punched out the side of the hard case and then the box, sticking out for the world to see. Thankfully it did not do any discernible damage to the barrel.
Insure it well and cross your fingers.
If an expensive rifle you could add the cost of a QUALITY hard case to the price. However, you will probably have to wrap the rifle inside the case to prevent it from sliding around - see above.