I tried to find one I've received to double-check, but couldn't so I'm making a semi-educated (but vague) guess that they issue them via one office when they go to the sender. The last one I had was ages ago, but have had 3 buyers since then contact me about having to pay to pick up after my stamps apparently peeled off in transit (but not my return address label, so they should have sent the notices to me  ).