isn't the extra the one listed on the toc as "What did you say?" which is included at the end of chapter 5's scanlation already, or is there a different extra that you're talking about that's listed somewhere else? (as far as I'm aware this only has 5 main chapters + the extra and digital bonus) but if there's more then I wanna buy it so I can read it.
Not sure tbh, but since MAL says it has 6 chapters, I’m gonna assume there’s more than the short digital bonus (the MAL mods only add physical publications to the database) I searched for it in Japanese, but I couldn’t find the ebook chapters listed out like usual :/ and the scanlation team sadly hasn’t gotten back to me yet.
I don't use MAL cuz they don't update their listings often enough but this manga most certainly does not have 6 chapters if it did it would be listed in the full volume purchase which it's not, it only lists the 5 main chapters + the extra (and digital bonus depending on which platform you buy it from), they may have counted the extra as a chapter the same way other tracker sites do since most sites don't count them as a half chapter.

NO, I NEED MOREEEE