This is how I do priority shipping. I shipped 5 shipments out this past week this way and I am pretty sure they all made it. I know for sure 3 of them took 3 days and are doing great. One of the people that got them 3 days later said that the zoas were opening within minutes.
Go to Wally World, go back to where they sale their plastic food containers. Look for the $2 half thermons that comes in red, blue, etc..
Take your frag bag it up with just enough water to cover it good. Then tie it up put in in the thermos. Add some peanuts or paper to make it tight. Screw the lid on good. Add the thermos to a 4x4 usps priority box. Line the bottom, the sides, and top with packing peanuts. Close it up and ship away.
This has worked MANY times for me. I can normally fit 3 small frags only in the thermos cause it is small. There you go.