In Myeongdong, Daiso Myeongdong Main Store (43 Myeongdong-gil) is the closest,

and there's also Myeongdong Station Store (134-1 Toegye-ro). Both are open until 10 PM. The heated eye mask (₩1,000) is single-use and warms up when you open it, nice after a long walking day.