jwolthuis - 1:37 pm on Apr 21, 2012 (gmt 0)
Maybe an address isn't valid to the post office.
I've found exactly the opposite. USPS delivers to remote Alaskan villages, to US territories (Puerto Rico & US Virgin Islands) as domestic mail, to PO Boxes, and to military personnel overseas. They also have the lowest rates for international shipments.
I have not yet had a customer to whom I couldn't ship to via USPS. The same cannot be said for private carriers.