Added another shelf, unfortunately I didn't spot this the other day but it seems that the 1x4 boards warped in the heat of the sun.

This actually presents an issue I hadn't thought of. Namely, if this unit is setup on a window in the north with an environmental control setup; the growing environment will likely have high humidity.

The 1x4 boards, unless they are treated will almost certainly warp.

It's a bit of a setback actually. I was hoping to have the bulkiest item shipped being the plastic planting trays with baggies of starter soil sandwiched in the middle. Pictured below is the second shelf added, and the board itself has actually warped significantly.
It seems to work, but it's pretty crude.

An avenue I may be looking at implementing is utilizing aircraft cable almost entirely for the structural support.

This would mean 3D printing brackets to hold the planter trays and a suspended adjustment system for levelling the planters. Such a setup might actually be the best setup, as it minimizes the shipping footprint to basically just aluminum wire and accompanying hardware. Items which could likely fit inside a single, low footprint envelope.