Comment by mjevans
"""
United Kingdom
The Royal Mail stopped selling IRCs on 31 December 2011[26] due to a lack of demand. United States
The United States Postal Service stopped selling international reply coupons on 27 January 2013.[27]
"""
https://en.wikipedia.org/wiki/International_reply_coupon#Uni...
That explains why I was confounded in my efforts to search within USPS results.