Cardboard is a pretty straight-forward material, with Diffuse, Reflection, and Bump maps. Keep the texture Blur values low, to avoid blurring the fine details.
The only notable thing here is the hand painted Normal map that allows you to control the amount of corrugation. Turn the Normal strength to 0 for a flat surface, and bring it up to increase the corrugation (5 in the example render).
Even in completely flat cardboard pieces, there is a subtle hint of corrugation sandwiched between the two flat sheets, so consider leaving the Normals at at least 0.1.
