Harry is exactly correct. This is why I've had to talk to a few shippers in the past who sent me shipments that were cold despite having heat packs. They sent solid styro boxes (not the pieced-together ones), which is fine, but then they taped all edges completely, sealing the box and heat pack, and thus, as Harry said, eliminating the heat pack's source for new oxygen.
Those heat packs can work just fine if given a small source of air. Reptiles and other small animals do just fine with these, despite being more exposed to outside air, because the heat packs react with this incoming air. In our case, the frags won't be suffering from a lack of oxygen. What I usually do is only tape the main seam of my boxes to ensure it's not coming apart. I'll even double tape it with overlapping layers. This still leaves opportunity for air to enter in small amounts. If I'm not using sheet styro, but a solid extruded box instead, I will poke a small hole (1/2") in the top of the styro lid, and I then tape the heat pack directly over this hole, so that all air entering must go through this heat pack. I also never tape the styro lid down. The pressure fit of the cardboard box is enough to hold the lid on. The heat pack should always be secured to the top, so the corals aren't sitting on the heat pack as the heat rises. I also often put a layer of newspaper or cardboard inside the box to creae a barrier between the heat pack and the corals. This helps to dissipate the heat in the box a little more indirectly.
For what it's worth, I use massive amounts of heat packs for shipping freshwater fish I breed and sell. The heat packs are much more necessary for the fish, and have to be reliable. For corals it's not as big a concern. I've had plenty of packages come in with mid 50's water and they were fine. Not so with fish. I prefer the uniheat brand and I use the 40hr and 60hr heat packs.
