I have taken large Priority Mail Flat Rate Boxes and lined them with cut styrofoam sheets bought from Home Depot. Put a chunk of dry ice into a bag, surround with shipping peanuts, and place a small piece of cardboard between the dry ice and the food (this prevents the vac-sealed bag from cracking due to contact with dry ice). Place your frozen items in and pack with shipping peanuts. Take directly to Post Office before noon. Items have arrived still frozen on the second day. Insulation is the key.
Do not ship on Fridays or Saturdays as they can sit over the weekend and spoiling your food and your plans. If distance is far I would suggest shipping Monday through Wednesday only, especially if rural delivery is involved.
Humphrey's DownEast Beast W/BBQ Guru