Customs Form for DPO address?

[NHAero]

Just came back from the PO, where I went to mail an item I just sold on ebay. Address is a diplomatic address and the PO person handed me a Customs form to fill out. He also said, once it gets into the military mail system we lose track of it. I Googled the buyer, it looks as though he is a Deputy Mission Director for the Army, based on Aurora, CO.
Do I really need to fill out a Customs form, which asks me for his email address, etc.?
Is it riskier to send to a DPO address, in terms of it not getting received, and no way to track it?
Thanks

[NHAero]

BTW - my Shipping Prefs on ebay exclude APO/FPO and this buyer's address is DPO, which I understand to be diplomatic