KoreaPost creates updates on Korean stamps for different social media platforms (both domestic/Korean and international) on an almost daily basis. One of the campaigns running at the moment is the one related to the new stamps showing the Kakao Friends characters.
This series is part of a larger series, spanning several decades effectively, showing Korean cartoon characters. Some of the earlier stamps are shown in the current campaign:
In case you’re wondering about some of these characters, here is one example from YouTube, the complete series of 아기 공룡 둘리 (“Agi Kongnyong Tulli”) or “Tulli the baby dinosaur”:
The original post can be found on Facebook.