Bag the frags, wrap everything in newspaper, put the heatpacks between the styro wall and the newspaper, you wouldn't want the heatpacks to touch the plastic bag, they get really hot and could damage the bag or cook the frags. I have also seen people tape the heatpacks to the top of the styro.
Depending on how cold the area is that you are shipping to, you might want to wait until the weather is at least in the 40-50's, that is JMO though.