Technically, the RSVP cards are rude. They imply that those you have invited to your event would be so crass as to not respond to your invitation.
So there is not etiquette violation in not supplying either the cards or the postage.
Okay, I know the tradition has changed. But I just wanted to point out the older tradition, because sometimes knowing the roots of a tradition can help you out with the newer version of the tradition.
I don't even know if people in other countries use the RSVP cards. In places where they don't, I suspect no one would care if there was postage on the envelope or not.
I wouldn't go nuts trying to get stamps from other countries. Instead of the RSVP card, I'd include something that gave an email address and/or website to respond to, and leave it at that. In effect, going back to the older tradition, but with the modern twist of electronic communication instead of pen and paper communication.