Question about shipping to a military FPO, AP address

I am enrolled in eBay International Shipping program. I sold a pair of boots that is going to a FPO, AP address. It is asking me to fill out customs forms. I've never had to do this before. I assume this is because it is a military address and is not going though the eBay hub. My question is who is responsible for any customs duties - the buyer or the seller? Thanks.