Thermally insulated shipping system for use in transporting a pallet-sized payload. In one embodiment, the system includes a plurality of thermally insulating walls arranged to define an interior volume suitable for receiving a pallet-sized payload. The thermally insulating walls include a top wall, a bottom wall, a front wall, a rear wall, a left wall, and a right wall, of which all but the bottom wall include at least two slots facing towards the interior volume. One of the slots is an inner slot that is more proximal to the interior volume, and one of the slots is an outer slot that is more distal to the interior volume. The system also includes a plurality of inner cassettes disposed in at least some of the inner slots and a plurality of outer cassettes disposed in at least some of the outer slots. The inner and outer cassettes include phase-change materials.