Edited to Add 12-Jun: There is some question regarding the authenticity of the official status of the merch. If there is anyone who can read Mandarin well, would you mind taking a look at the links?
A user recently posted an unboxing video of the box-set, which was then shared on Twitter here. [Note: the original video was likely on Weibo (analogous to Facebook / Instagram), which restricts posts to users, hence the repost to Twitter.]
Edited to Add 10-Jun: I mixed up two box-sets! The one with the concept art and stuff (in the twitter unboxing) is a separate box-set, found here. This box-set includes official(?) concept art, a book of poetry references used in the script, and official(?) promo art.
A separate box-set includes WKX’s fan, WKX’s hairpin, bookmarks, postcards, acrylic standee, and wallet. It all looks gorgeous, but at the moment, I can’t comment on the quality of any of it.

( more photos behind the cut )
The source of the photos is Taobao, a "consumer-to-consumer (C2C) retail by providing a platform for small businesses and individual entrepreneurs to open online stores that mainly cater to consumers in Chinese-speaking regions (Mainland China, Hong Kong, Macau and Taiwan) and abroad." (Wikipedia)
This article explains how to make a purchase on Taobao as an international consumer.
If anyone has more details, please feel free to add in the comments, and I’ll edit my post accordingly!
A user recently posted an unboxing video of the box-set, which was then shared on Twitter here. [Note: the original video was likely on Weibo (analogous to Facebook / Instagram), which restricts posts to users, hence the repost to Twitter.]
Edited to Add 10-Jun: I mixed up two box-sets! The one with the concept art and stuff (in the twitter unboxing) is a separate box-set, found here. This box-set includes official(?) concept art, a book of poetry references used in the script, and official(?) promo art.
A separate box-set includes WKX’s fan, WKX’s hairpin, bookmarks, postcards, acrylic standee, and wallet. It all looks gorgeous, but at the moment, I can’t comment on the quality of any of it.

( more photos behind the cut )
The source of the photos is Taobao, a "consumer-to-consumer (C2C) retail by providing a platform for small businesses and individual entrepreneurs to open online stores that mainly cater to consumers in Chinese-speaking regions (Mainland China, Hong Kong, Macau and Taiwan) and abroad." (Wikipedia)
This article explains how to make a purchase on Taobao as an international consumer.
If anyone has more details, please feel free to add in the comments, and I’ll edit my post accordingly!