ice
Mvite,
I always use ice packs when shipping to an area that has a day time temp of 82+ degrees. In the past when I was still learning to ship well I didn't and had a lot more doa's.
I usually check my customer's zip code on weather.com and do the hour-by-hour report. This will give you a good idea of what conditions you need to pack for, instead of going into it blind. I used to think doing this was too much of a hassle. But now I know better and it takes a lot of the guess work out of shipping.
When shipping larger corals, I wrap the ice pack in a sheet of newspaper then bag it and then bag the corals and it together (hope this isn't confusing).
For frags or smaller corals I usually bag up the frags, then place them in a 3mil bag alone. Then I take my bagged ice pack and place it in the box with them. That way the ice pack keeps an ambient temp in the box without chilling the actual frags much.
I don't know for how long a capri sun stays frozen, but the ice packs I use probably stay frozen for maybe 4-6hrs wrapped.
Using ice or heat packs when shipping corals is really more about maintaining the same water temperature throughout the shipping process instead of trying to "cool" the water the corals are in. I've shipped to areas with 100+ and -10 below temps with success.
I've found that shipping corals requires a lot of trial and error, but I'm pretty happy with the system we've worked out now.
If you have any questions let me know. ;]
.. cuz he told me so! 
