When I ran my own company in Maine for about 5 years we shipped year round.
Here are a few tricks we used to make sure everything got there nice and warm.
We used standard superior boxes with 3/4" foam to ship with. We put 2 small air holes on each of the arrows on the box with a 16 penny sized nail to allow air to get to the heat packs.
One 40 hr heat pack for a 12x9x6 box, and 2- 40 hour packs for a 16x16x9 box.
Each box got a layer (about 1"-1.5" thick) of newspaper that had been crumpled into a ball them straitened out to fit into the bottom of the box. This allows some air flow but also adds more bulk layers to it to protect from being placed on a cold cement floor of a loading dock. This also allowed more bulk so if cold air did get threw it had more bulk to get threw than reg. flat paper would offer. The animals were then placed inside the box in the deli cup, and more paper was wrapped around the out side of the cup to fit just below the air holes on the deli cup. From there all the empty space in the box if filled with the same paper until it was full. If using multiple cups, fill the gaps with paper as well. From there a few layers of the same go on top of the cup to cover the box with a small corner on each end not completely covered. The heat pack (5 hrs to warm up properly) was them wrapped in one layer of reg. newspaper with ends open to allow heat to escape, and taped to the lid. This protects the pack from any moister that could cause it to short out. Top of the box gets three stripes of tape to seal it,same as bottom (one to close and one of each side to keep the cold air out. We shipped around 300 boxes a year this way and never lost a animal due top the cold. More than half of those boxes were shipped in temps between 15*-25 * as a low.
If using a snake bag, same way as a cup, just fill in any voids with thick layers of paper and you are good to go.
The purpose of this is to make the box insulated enough that if the heat pack is defective you are still covered.
We have shipped anything from baby colubrids and rainbow boas, up to $10k ball morphs like this with no issues at all.
If at all possible, snake bags work best even for babies as they do not retain the cold like deli cups do.
I have received animals packed like this as low as 0* out when we have had a temp drop unexpectedly that other than being a little chilled were fine after a few hours warm up.
Don't be afraid to use to much paper in the box, as that is what saves the animals when the unexpected delays, or temp drops happen.
Hope this helps, as it has worked very well for me.