When I started this adventure, I discovered that there was surprisingly little information out there about how to ship my breastmilk. I prefer the lansinoh bags when shipping because they lay so flat. I believe that you can ship with styrofoam coolers, but I don't know how the cost works with that. Instead, I had a shipping cooler built by UPS. The provide a box and cut styrofoam to fit on all four sides and charge about $15. I typically visited between shipments so I took home the box and reused it. If you aren't able to do this, it may be cheaper to buy shiping coolers in bulk online- that was the only other place I found them.
I shipped between 200 and 290 ounces most times and needed a 12x12x8 (I think) box, which typically weighed about 20 lbs total. I included two gel ice packs because they stay colder longer and don't make a mess. Because I shipped within state, I used UPS ground and was guaranteed delivery the next day. This typically cost around $20. If this isn't possible, the milk should stay frozen for two days to ship via 2-day air and save money over overnighting it. We did this once and it worked just fine. Make sure that the milk is frozen solid. When I shipped some that wasn't, it busted and froze the milk bags together (no fun!). You will also want to put ice packs on the top of the package as cold sinks to the bottom. I like to put one on the bottom to be safe as well.