There are several night shopping locations in Seoul. If you want to shop for cosmetics, the Myeongdong area is the most suitable place because in this region there are so many famous cosmetic brands and hundreds of shops where each brand can have more than one store. The experience of learning cosmetics in July (summer) is a perfect time because this month there are so many discounts. My advice for learning cosmetics in myeongdong should be able to compare prices first because don't regret the items you buy are more expensive. My advice for shopping for cosmetics at Myeongdong is cheaper, you can get it in the underground shopping center to Myeongdong Station. there is one store that sells cosmetics and is quite complete at a cheaper price. So if you know what you want to buy, you can compare prices in the store.
If you are looking for fashion products, you can go to the Dongdaemun area located near the Dongdaemun design plaza called APM (open at 8pm) and the surrounding area. When I was there at the weekend and was very lucky I found one area (if you are on the right side of Dongdaemun Plaza, go straight to meet traffic light turn right, straight then) like a shocked market and seems to only open on weekends. There will find a shopping paradise for clothes that are sold very cheaply (when I get the price of one men's formal dress for 1000 won). Goods sold starting from clothes, pants, sportswear, shoes, sandals, and so on. So don't regret if you buy clothes at the mall then shop in the market.
For fashion shopping at low prices you can also find in the area around the university such as Hongdae University, Ehwa University
My advice is if you go to Seoul in July, cosmetic shopping is the right time