Comment by Symbiote
That's the International Reply Coupon mentioned in the article, but it's not supported by all countries.
I think I've sent far more international letters and parcels than domestic. Christmas cards for elderly relatives in the country I was born in, and postcards when I travelled abroad.
Some obscure things I sold on eBay were mostly sent abroad.
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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]
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https://en.wikipedia.org/wiki/International_reply_coupon#Uni...
That explains why I was confounded in my efforts to search within USPS results.