The heat packs can actually help to melt the plants.just fyi ... I got all my plants to boston last and 2 weeks ago. I had no mishaps except for the package with the heat pack. As soon as it's nicely wrapped you shouldn't need a heat pack. Just my 2 cents. At least for plants!
As long as the package doesn't end up sitting in a freezing cold mailbox for a day or so most plants should ship fine. Plants should also be packaged in a damp/humid environment but not overly wet... the water will freeze before the plant tissue.