In Myeongdong there's Daiso Myeongdong (43 Myeongdong-gil) and Daiso Myeongdong Station (134-1 Toegye-ro). The handle outdoor insulated bottle 650ml (₩5,000) is, as named, an insulated bottle, nice for warm tea on chilly days. Empty it before your flight.
Daiso Mall
Daiso MallDaiso Mall