Ditto to this. I'm in Australia as well. Have had a PO Box for years. USPS is no problem. I think some of the FedEx type couriers don't deliver to PO Boxes. I have NEVER had any problem with USPS. Especially with their tracking services. I have business partners in the States and receive several packages every month.
Thought I'd just add this image. I've always worn my 3706 on a Zulu or Nato. (the Nato is a very dark blue).
The biggest problem with Nato's is that once you've worn them, anything else doesn't seem as comfortable. I love 'em and they look good as well.