I'm about to send a few portions out to friends on here...here's my recommended shipping method:
Get some of those ziplock containers, they're cheap, about 5 bucks for 6 of them.
Fill them 7/10 with water, drop the caulerpa in, seal the top, then tape the top securely with shipping tape. Put it in one of the US Priority mail small boxes they provide.
With this method you can ship using US Priority mail which is only 3.95 anywhere in the US.
Works!