Well, I'd still be very hesitant to just send it in. How does Joe CustomsAgent looking at your box know that it's coming in for warranty service? At the very minimum, you should be very careful to address it to:
Benchmade Manufacturing Company
Attn: Warranty Service Department
300 Beavercreek Road
Oregon City, Oregon, 97045
USA
Contains merchandise being returned to manufacturer for warranty repair.
Be sure to note on the customs declaration in big, friendly letters that the contents are being returned to the manufacturer for warranty service. And, I'd also enclose a simple letter, unfolded, unsealed, that reads something like:
Dear Benchmade Manufacturing Company Warranty Service Department,
Please find enclosed my Benchmade Model 42 knife. Please note that the screw holding one of the handles has broken off. I have marked the broken screw by applying a bit of red tape to the handle near it. Please repair this broken screw according to your standard warranty and return to the knife to me at my address in Oslo, Norway given above.
But, even with all of these precautions, I'd still contact US Customs (they've got a website and you can e-mail 'em) and ask for a waiver. That way, there's just no question. Without the waiver, I fear that if something does go wrong, you won't be able to salvage the situation. With the waiver, if some mistake is made and your knife ends up in line for the crusher, you've got some documented paperwork to stand on. Since it is a warranty return situation, I don't think customs will have any problem granting the waiver.