The one thing to keep in mind about FedEx is that their airplanes hold are heated (to 80 degrees or so), so are their trucks (to a lower degree). The warehouses are heated as well...so there are only a few times a priority overnight package might be exposed to really cold temps (waiting when the truck is loaded, airplane unloaded and going from truck to door). Just some info to ease your mind a little about shipping in colder temps.

But you should still do a test box with one of those min/max temp gauges to see how the box does at just holding the temps fairly steady (checks your insulation)