You might think it would be fairly simple to 1) determine if a computer-generated USPS customs form doubles as an address label or 2) find a simple quarter-page from/to address label template, but you would be wrong.
Clearly I'm going to end up having to design my own in Scribus or something but that is a problem for tomorrow-me.
@cocoa things I learned my last visit to the post office: if the to and from sections start at the same line height, their computer will apparently mix them up and send it to the sender.