Insulated box...styrofoam box inside a cardboard box with a 48 hour heat pack or longer if your not doing next day shipping, but not right on the bag the corals in. Maybe place a piece of styrofoam inbetween the two. Double or triple bag so it doesnt leak because if it leaks on the heat pack the heat pack will no longer work. Make sure you mark the box with live animal and and arrow pointing up or they will throw it around like a dish rag. Anyone else have more pointers?
I would only ship overnight in the winter. I've lost a few shipments that didn't make it because of shipping. It's just not worth risking 2 day. Package as late as possible as with any corals and watch the weather for any storms or possible delays.