All packages shipped to an APO/FPO address require a customs form. You DO need to list the entire contents of the package. You can fill out a customs form online at usps.com. Check with your local post office for their guidelines.
Your local postal facility will give you the customs forms to use. Ask them if you can take a stack home with you to save time at the counter the next time you come in to mail a package to your Marine.
To order the forms online (at no charge to you), go to shop.usps.com and enter "customs form" in the search field.
Some post offices require the "contents" section to be detailed; others allow a short description. Again, check with the post office you will be mailing your packages from.
Eight Areas to Complete on Customs Form: