7x7x6 size box with 3/4" thick insulated styrofoam. Styrofoam acts as insulation to keep you hatchling warm or cooled, depending on the season. It also prevents the box from collapsing or being crushed. I get my boxes from shipyourreptiles.com.
Two 32 ounce deli cups, 6.75" diameter. Make sure it has pre-punched holes and a lid. For most hatchlings you can get away with a 32 ounce deli cup.
For a 7x7x6 insulated box, you only need one heat pack. Do NOT use hand warmers or feet warmers for humans. These get way too hot, and they don't stay consistent in temperature. Make sure you use 40 hour UniHeat packs.
Moss is the substrate that your hatchling will travel in. It holds moisture pretty well, and acts as a nice cushion. I have been using this terrarium moss for years and it has worked great.
I use newspaper and sometimes paper towels.
Don't forget the tape. I can't tell you how many times I've started to prepare for shipping a hatchling and I don't have enough tape.