I understand the "one letter" thingie; it's the USPS bit - what does it mean? I look at that and say United States Postal Service which I am sure has nada to do with it.
Charlene got it. To be honest, it doesn't really matter since the clues are answerable without the USPS caveat, but what that does is give you a free "answer list." That is, if you can't spell your answer using 2-letter USPS state abbrevations, you know it's wrong.
States only- no provinces (Sorry Charlene!) or possessions. So something like the product GU/MO/UT (GUam/MissOuri/ UTah) I won't ask about, nor a really cool one like VI/CT/OR/IA (Virgin Islands/ ConnecticuT/ORegon/IowA)