During the so called "overnight" shipping, the sellers don't put in heater or battery powered power airpumps in the insulated styrofoam they put the corals in.
Not to mention, "overnight" only means that the package will be on flight overnight but sits in the office for 24 hours or more.
Here's a good article that should be inspiring
http://www.advancedaquarist.com/2003/2/short