Olive Young Myeongdong Town (53 Myeongdong-gil, 1F-2F) is open until 10:30 pm,

and Daiso Myeongdong Main Store (43 Myeongdong-gil) nearby until 10 pm, so you can do both. Masks from Olive Young, tools from Daiso is the cheapest combo. The Daiso pack spatula is ₩1,000 with 6.6K+ orders, handy for spreading the mixed mask and reusable after washing.