An Shoe box flatpack can be made at any workbench in the workshop of a player-owned house. Like when making the normal Shoe box, this requires 21 Construction, uses 2 plank, 2 nails, and gives 58 experience. The flatpack can then be used on a wardrobe hotspot in a Bedroom, either in building mode or not. This requires no Construction level and gives no experience.
The cost of making and selling a Shoe box flatpack from scratch gives a profit of: -991. This assumes the nails did not break, and steel nails (highest nail available at sawmill) were used.
After completing the hard Fremennik Tasks, they can also be traded to Advisor Ghrim in exchange for 39 coins being added to your kingdom's coffers in Miscellania. See Flatpack: Miscellania reselling for details.