Todd Wheeler talks about hunting the fabled International Reply Coupon, one of those mythical beasts like the Loch Ness Monster or Bigfoot that people pretend exists but really doesnt.
I have had similar nightmares with IRCs. I was trying to send a query to a British agent that I really admire. The people at the post office near my house insisted that IRCs no longer existed, until I finally showed them a printout of the Post Office web page. Then they tried to claim they actually just said they were out of them.
Worse, if your manuscript/query has any heft, there's no real way to know how many IRCs you need. And then the fact that an IRC isn't like a SASE that you can just drop back in the mail. They have to go down to the post office and cash them in.
I think the solution is to mail a disposable manuscript with your e-mail for their response. Anyone who doesn't see that it's far more convenient to just e-mail your rejection (or, God help me, acceptance!) than to go down to the PO to redeem an IRC is so insane you don't want them looking at your story.