I ship all year round. I use insulated boxes and heat packs.
Keep in mind, heat packs on average raise the tempearture about 14 degrees within a cubic foot. Please remember as well, the heat packs need oxygen in order to continue working and do not work below a certain temperature. If you are using a pre-formed styro, you need to poke a hole in it so that the heat pack can continue to work, otherwise it depletes all the available oxygen in teh box and goes cold until the box is opened. Also you need to pay attention to which side the red stripes are on and make sure they are not sandwiched against the box/styro lid. These should be facing INTO the box. Use insulation to protect the bags from uneven heating (newspaper works well). You can also "jumpstart" a heat pack by putting it in a bag with some oxygen/air to get it warm immediately for shipping. Keep in mind, this lowers the length of its use. Heat packs do have a peak temperature, so you need to be sure you do not overdo it with them.
Also keep in mind, you don't account for outside temperatures as they are in heated trucks.
I jsut require a signature for fish so that I know the package will not sit outside, or do hold for pickup.
Priority packages must have someone available to receive them when temperatures are below freezing.
I do take of about 3 weeks in December (2nd week of december to first week of january) as there are unavoidable postal delays no matter what (express shipping I still do then)