I purchased some IRCs from the USPS online site and received them unstamped.  I sent them off with QSL requests.  

Today in the mail I received replacement IRCs, stamped nicely in the left box, with a letter.  Excerpts from the letter: "The International
Reply Coupons you originally received were not properly postmarked and therefore, cannot be used for mailing.  These may be discarded.  
We are enclosing replacement International Reply Coupons, postmarked on the left side(under heading "control stamp of the country of origin"), as required by US Postal Regulations."

Interesting.

Jeff, W7JFF