All packages shipped to an APO/FPO address require a customs form. You DO need to list the entire contents of the package. On the customs form, check the box for "bona fide gift". Click here for example of new January 2004 form.
The new form (January 2004) is multi-part and you will get a copy for yourself. 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 your 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 2976 in the search field. Your search should result in three products being returned to you (see image below). You'll want the 2nd form which is multi-part. The third item is the plastic sleeve that the customs form should be inserted into.
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.
For additional information on this guideline, see article B1 of the USPS Restrictions as posted in the Postal Bulletin 22127. Click here for example showing 8 areas to complete.