Wiktionary
flat pack
a. 1 (Typically with regard to furniture) able to be constructed from flat pre-cut sections. 2 (context by extension English) easily constructed from pre-produced components. n. (Typically with regard to furniture) the pre-cut kit of sections used to make a piece of furniture.
Usage examples of "flat pack".
Then, using a flat pack of slim steel files from his top pocket, he started to work on the softer metal of the skeleton key.
Carmady brought his eyes back, reached out a flat pack of new hundreds, riffled them.
It's like saying 'Flat Pack Easy Assembly' or 'Military Intelligence'.