I live in New York. I ship fish. Many years ago I sent a box of LF bristlenose plecos to a buyer in Alaska in April. Back then I used the USPS to ship. For some reason the box I sent out on Monday afternoon did not arrive in Alaska until Friday morning. All the fish were alive.
I have no problem shipping fish in freezing weather. that is what 1 inch styro and several heat packs are for. However, I will not ship after thanksgiving until the middle of the following January. This has nothing to do with the weather, it has to do with the volume of shipping related to X-mas. Boxes get bumped all the time and the shipping companies do not care what might be in a box chosen for delay.
What I have refrained from doing is shipping in summer when it is extremely hot. his requires cold packs. I do fine with heating during cold but am to unsure of things when it is the reverse. Most of the fish I ship are from warm water. I keep them in the low- mid 80sF. In shipping my goal is to insure the coldest the water gets is about 76F. I know how many and what size hear packs to use. I have no c.lue about using cold packs in summer.
Also, I know that if one has fish at a temperature, high or low, which will soon be fatal, the way to save the fish is to get it into the right temperature range ASAP. If one bothers to try and acclimate them slowly, they will mostly die. I have first hand experience that confirms this. I learbed do this because a few years backwhen I was looking for research papers on acclimating fish. Everything I found dealt with temperature studies for determining the specific lethal temps for specific species.
Most of the papers wanted to find the fatal point which meant that the fish died. But, another part of the research was to learn how the fish behaved as they approached the fatal temp. One group of scientist decided they wanted to know if, when the fish exhibited that behavior, could they then be saved. What they learned was yes they can, and the way to do it was to get them into the right temperature water as fast as possible.
I do not acclimate new fish. When they arrive the bag is opened and the fish go into the Q tank directly (but not the bag water). If there are several bags of the same species I will dump the fish and bag water into a bucket. From there I net them and plop them into the tank. I want fish out of bag water as fast as possible.