>>postpaid<<

It seems to me this term has been used to indicate an envelope or return-envelope that has the postage prepaid
by the business desiring your response. "Just drop the postpaid card (or envelope) back in the mail," or "Just use the postpaid envelope."

And hearkening back to the "hyphen" thread...I think I prefer post-paid (and have seen it that way many times before)...but, ahem, that's just me.