US customs goes by whatever the federal law is on the importation of controlled substances. Since it is illegal, it's really not gray, just not enforced zealously (aside from seizures) because of lack of funds, manpower, and hopefully better things to do most of the time. I will say they aren't consistent in this at times since I used to hear regular stories about people receiving controlled meds that had been opened and sent on, but in those cases it was probably clearly a personal use quantity and whoever opened it didn't feel like doing the paperwork. This doesn't seem to happen much anymore if the substance is identified as a controlled drug here, whether it is sold OTC in other countries or not.