A modular box assembly includes a box defining a box cavity within the box, the box further defining a top end and a bottom end, the top end defining a box opening connected to the box cavity, the box including a side panel extending from the top end to the bottom end; a shoulder attached to the side panel, the shoulder extending inward from the side panel and into the box cavity, the shoulder including a first sub-shoulder and a second sub-shoulder, the first sub-shoulder, the second sub-shoulder, and the side panel defining a shoulder channel; and an inner box disposed in the box cavity, the inner box including a channel tab, the channel tab frictionally engaging the shoulder channel to secure and suspend the inner box within the box cavity between the top end and the bottom end.