You may be ahead of me in this story, deciding what you might do with the postage-paid card, but wait! Before you drop that card in the mailbox in you head, let me tell you that each card included the warning: “mailers of blank cards will be prosecuted”.
Ooh. Now there is a challenge. Wouldn’t want to tempt prosecution by mailing in a blank card. They might come straight to your blank address and arrest right you on the blank spot.