Besides airline perishable embargoes, it completely depends on the shipper. There are shippers that won't use more than two heat packs in a box no matter where or when. We often use 4-6 in winter to cold areas depending temps. Good shippers watch the weather closely and know what and how to do it right. I've shipped Alaska, MN, NH, etc. in Jan. and Feb. without issues. We use 40+ hour heat packs, not the one day cheapies, in case something gets stuck on the way, especially important for connection flights. The airlines are often overly sensitive to freezing with their embargoes. Heat packs can't get wet and have to stay above certain temps or they quit burning. Besides a delay, a loss due to cold is simply a shipper that doesn't know what he's doing. A heat pack raises temps in a box about 10 dF per heat pack, use 6 long-burn ones and there are no issues. Yes it costs $5-10 more for the shipper, and this often sadly is why there are cold losses. Not because it can't be done. A number of shippers in LA ship every week, everywhere, without issues. Then lots of others have problems. Some have problems in good weather. It is not the air temps.
warmly,