The above post is about posting FROM the US.
Posting TO the US may be (and probably IS) different - they are far fussier about what they let into the country than about what they let out or allow to be sent within. You've customs to cope with as well as mail regulations
From US customs website
The entry of prescription medicines is restricted and subject to the approval of the U.S. Food and Drug Administration (FDA). Depending on the FDA review of the medicine, it may be released to the addressee or seized