List of Seoul post offices open on Saturday from 9 am to 1 pm. If Saturday is a holiday then the post offices are closed.
http://english.seoul.go.kr/lh/residence/cnb.phpFollowing is a list of post offices that open on saturday (until 13:00):
Seoul Central, Gwanghwamun, Seodaemun, Dongdaemun, Gwangjin, Gangbuk, Mapo, Gwanak, Eunpyeong, Yeouido, Gangnam, Geumcheon, Seongbuk, Gogang, Songpa, Yangcheon, Gangseo, Dongjak, Seocho, Yongsan, Nowon, Jungnang, Seoul International, Yeongdong, Dongseoul Mail Center