Finally releasing the Doombox outfits individually last week was a bit of a bumpy ride!
Turns out granting the Doombox DLC to all existing Soundtrack owners made it impossible to purchase the new bundle (or the Soundtrack, when owning Doombox first).
To resolve this we removed Doombox as a bonus outfit from the Soundtrack, but everyone who purchased it before today still owns the outfit individually.
If you had any trouble completing this new bundle before, it works now!
https://store.steampowered.com/bundle/16023/Soundtrack__Outfits_Bundle/