Regarding IRCs - In my experience, I have found that publishers don't like IRCs much. Are they any trouble to redeem, I wonder? I have sometimes had them returned to me. (I am way over in Australia.) Plus not all postage outlets sell them any more.
Ordering stamps via US post sounds like a good option.
By the way, I don't know if this is really acceptable but, when US publishers ask for return postage now, I advise them in my cover letter to let me know their response by email and to please dispose of any rejected material in the recycling. I haven't had any complaints yet, although I have only tried this once or twice.