I just received a package from Peter, and despite 90-degree temps, all six grubs I ordered made it (including 4 Goliathus!!!). The package had three layers. The first two were nested, small Priority Mail boxes. One was slightly larger, but they fit together tightly (I can measure them when I get a chance). These two nested packages were put in a large Express envelope next to an ice pack. The grubs were placed in cups of flake soil (even the Goliaths).
I also sent some roaches recently in similar temps, and they were in a 12 x 9 x 7 Priority Mail box. The bottom of the box had a piece of high-density polystyrene insulation foam with a paper-wrapped ice pack on top. The roach container went on top (70 B. giganteus with slightly moist paper towels in a large, rectangular Ziploc Tupperware that had about a 2-in x 2-in mesh ventilation hole in the top) and it was also wrapped in multiple layers of paper. The remaining space was filled with reused packing peanuts and I put a piece of Styrofoam on top. All the roaches made it with Express 1-day shipping.