Coupang
When it comes to internet buying in Korea, Coupang is even better than Amazon. Coupang is a shopping website and mobile app. They have been so successful in part because next-day delivery is guaranteed for all orders placed before 11:59 IST. The fact that it is primarily intended for native Korean speakers may be off-putting to those who aren't fluent in the language.
App: iOS/ Android
Website: https://www.coupang.com/