I'm assuming he's military. I don't know about his unit's regs, but heres the postal rundown:
You'll be technically shipping it domestically, and paying domestic postage. Army to NY, Navy to California. You'll still need to fill out customs form, 2976-A It's a white form, with about 6 carbon copies. (
https://webapps.usps.com/customsforms/# ) Fill it out with a detailed list of the contents, both addresses, and sign and date the bottom. Most of the right side of the form you don't have to worry about. Take this form loose with the package to the Post Office. The clerk will attach it to the box after finishing it.
I strongly recommend priority shipping. Prices are good, and generally so is treatment. There are free boxes available as well. The large flat rate boxes to any APO/FPO are discounted to about $12, and up to 70 pounds. I don't recommend jamming that much into one. But you can fit quite a bit into them.
As for the knife: (from pub 52, Hazardous, restricted, and perishable mail)
443 Packaging and Marking
443.1 General
All sharp–pointed or sharp–edged instruments such as knives, tools, ice picks, razor blades, etc., must be securely packaged in a strong container as required in DMM 601.1–8. Sufficient cushioning material must be used to protect the sharp points and edges from cutting through the outer packaging during normal Postal Service handling.
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Basically, have it locked into it's sheath, and packed it a manner that it won't break free.
As far as advice for APO/FPO mail in general, give yourself a good leadtime. I hear back that most things are getting to Iraq in about 2 weeks or less. The post office ships it to the Army/Navy, and then they take it from there. They pack the boxes into ANY available space. This space may not always be as large as the box being placed into it. Plan accordingly, use a lot of good tape, and be totally comfortable kicking the box down a flight of stairs. And then sending a wheel barrel full of bowling balls down after it.
Good Luck, if you have any other questions, lemme know.