Olive Young ships to the USA and to most other major markets, so you can order from home and skip the flight entirely. What you cannot have shipped is a tester, a 1+1 shelf tag or the tourist tax refund, and those three are the whole reason buying in Korea still comes out cheaper per item.
The rule I would give a friend: if you are visiting anyway, buy the bulky and heavy things, anything you want to feel on your own skin first, and anything sitting on a 1+1 tag that you already use. Order refills online afterwards. If you are not visiting, order online and stop reading shopping-haul videos that imply you are missing out. The gap is real but it is measured in a few percent and some free duplicates, not in half price.