I've read the uniheat gets up to 100F.
In my experience that does not happen. I've used the 60 hours for shipping; now the 72 hours. Shipped 4 boxes in September/October 2013 using the 72 hours. All 7 geckos arrived in fine shape!
In October 2014 I shipped 2.0 two year old
Oedura castelnaui overnight to Missouri via Reptiles Express. Both geckos arrived fine.
I enclosed a min-max Taylor digital thermometer while I traveled to the FedEx Ship Center in Portland in October 2014. Maximum temperature when I opened the box to remove the digital was in the 80s. That was 5 hours after activating the heat pack.
I use the 72 hour UniHeats all the time for shipping to provide extra insurance in case the geckos meet some delay. I activate the heat pack about 1/2 hour before sealing the 12 x 9 x 6 inch box. The usually unwrapped heat pack is placed right next to the deli cup or between two deli cups. The box has a single hole through both the cardboard and the styrofoam for a little ventilation. Top and bottom box flaps are sealed on each side with 2 strips of tape each rather than completing sealing off the flaps.
Before I shipped for the first time I ran tests in a cool interior room with the box sealed up as if to go (without the geckos).
In a 24 hour period the temps inside the box only exceeded the room temps outside the box by ~10*F.