Third Oak is challenging millennials’ perception about sustainable “Made in USA” footwear, one flip-flop at a time. The new direct-to-consumer brand is the millennial-focused spin-off of Okabashi, the Georgia-based sandal manufacturer that has been producing footwear in the U.S. since 1984. The collection, retailing for $30, launched Tuesday with women’s. Men’s styles are slated for…