So If you do a shop search for the Mysterious Costume Trunk, it says : "This item can be found in these locations: Achievement Prize". I'm having trouble actually figuring out which achievement gives this out as a prize, though, does anyone happen to know? Or am I stuck buying one for 50mil in order to get the achievement for opening one?

You get it from Filled Wardrobe for having 5000 items in your wardrobe. c:

Ahh thank you so much, I already have that achievement and sure enough, my trunk is in my vault lmao.

Hee hee, good for you!

Yep and you can get 500 Csc from it XD

Honestly I just need to open it for the achievement, I never opened one before they switched ii because I was so indecisive about which trunk I wanted, so getting CSC is ideal for me! Finally gonna do it haha.