In Myeongdong, go to Daiso Myeongdong Main (43 Myeongdong-gil) or Myeongdong

Station (134-1 Toegye-ro), both 10am–10pm. Finish with the glossy coating makeup fixer, 100ml, ₩5,000 (3.3K+ orders). It's exactly 100ml, so for carry-on, put it in your 1L bag with other liquids.