A Black Amex, flip-flops, and this "outdated" pant trend—introducing Mary-Kate Olsen's summer in the Hamptons...

      The Hamptons season has officially begun. This means that by 2 p.m. every Friday, New Yorkers start making their way "out East"—to the realm of Round Swamp Farm, blooming hydrangeas, and $75 fitness classes. Mary-Kate Olsen is at the forefront of the fashion scene. This week, the Bridgehampton local and co-founder of The Row was spotted carrying a brown paper bag and an Amex black card during a grocery run. While Olsen, who has established her career based on her stylish appearance, made the bodega bag look fashionable, it was her outfit that really stood out: FreeCity sweatpants and flip-flops. (Image credit: Matt Agudo/INSTARimages.com) On Mary-Kate Olsen: FreeCity Sweatpants ($168); The Row sandals For anyone else, this outfit might seem like a throwback from 2006. However, on Olsen, it was a perfect example of understated luxury mixed with nostalgic normcore. She maintained her typical loose silhouettes, combining the sweats with an oversized tee. But the true detail? Those sweatpants weren't merely a random throwback; they were a deliberate choice. FreeCity sweats are actually available at The Row’s boutiques. In essence, these sweats are less of a “dated” piece and more of a wardrobe essential that carries the highest mark of approval—Olsen’s endorsement. Then there were the flip-flops. Once limited to poolside and pedicure salons, the simple thong sandal has emerged as summer’s leading footwear trend. On Olsen, when paired with loungewear and a relaxed demeanor, it created an ensemble that subtly communicated, "I summer in the Hamptons, but I couldn't care less if you notice." Inspired to emulate her chic yet relaxed style this summer? Keep scrolling to shop the best FreeCity sweatpants and flip-flops. My favorite black flip-flops. I wore them throughout the Hamptons last weekend. Style them with a white tee, black sandals, and a raffia tote bag. I'm 90% sure this is the style Mary-Kate wore. The splash of green against the dark pants is so fun. Reformation Jessie Thong Sandals Fashion influencers on Instagram swear by these sandals. Pairing these with a white button-down shirt would create an unexpected yet cute look. Chocolate-brown leather flip-flops can elevate any sweatpants to look pricey. This colorway is so lovely. J.Crew Wide Thong Sandals Senior fashion editor Eliza Huber owns these in white and frequently raves about their comfort.