^^^^
I used a flat rate box to mail a piece of railroad track to Guam. The recipient uses it as an anvil for pounding out knives. It was light as far a flat rate boxes go, it came in at about 40 pounds IIRC, the track piece was nearly 12" long. All they had me do was tape it up very securely with duct tape and then covered it with priority mail tape. It got there just fine, which surprised even me.
bob