Just thinking of how I have shipped heavy stuff.
Like a bunch of knife blanks to the heat treatment folks, and steel to friends.
I used to mail steel from Crucible's distribution point in Arlington Texas to a buddy in Virginia.
(He quickly got tired of wearing out his belts on S30v and went back to other stuff.)
Typically I wrap everything individually, in paper or cardboard, then I build a wood box of scrap wood I have around. The wood box isn't that special. Just something tacked or screwed together out of what I have around the house. It has to be rigid enough to withstand being crushed. Also, I've used double ply cardboard and wooden sides before and tacked it all together with cheap furniture tacks before. The wood was more for a frame to tack the thick cardboard to.
Then on top I wrap all that really well in packing tape, a dispenser will pay for itself quickly, making sure that I leave a way for the person on the receiving side to get into it.
Finally wrap it all again in brown paper, or place it in the flat rate box and bubble wrap it to make it snug in the box it is actually going to the destination in.
Part of the value is the actual value of the box.
Part of the value is how hard you have worked, and the number of hours of labor, and the uniqueness of the part that you are shipping.
The first can be insured. The second cannot.
A good box can make a big difference.