Hi since this past april or so I have mostly been shipping moss around the country. Places include Florida, Maryland, Hawaii, New Jersey, and lots lots more, Even Singapore.
What I do for shipping depends on the place it is going.
For Florida customers (had a bunch over the summer) I will ship in a freezer grade zip lock baggy with about a teaspoon of water, as much air removed as possible. They are then put in a bubble filled envelope and shipped usps priority.
Closer states to RI like NJ, Delaware, Pennsylvania and closer depending on water I usually just take the ammount out and put it in the zip lock baggie. suck the air out and send them in the padded envelope using priority.
Hawaii I sent a bunch of Stringy moss so I used a large freezer zip lock bag filled it with moss and got as much air out as possible sprayed the bag with some tank water. but it in a padded envelope, this person decided to use overnight shipping, said moss came great!
When I ship things like pieces of moss on rocks and stuff like that I usually spray them down with tank wanter then put some white papertowel around it, put the piece in a freezer zip lock baggie and then spray the inside of the baggy. Idealy there will be a little bit of water on the bottom so the plants shouldnt dry out.
I would recomend checking the weather for the area, if it is going to be hott I include more water to try and keep the moss wet. If the place is close, I dont really bother.
It really just depends where you are shipping and what you are shipping I guess, there are lots of diffrent ways!
good luck shipping plants, as of date I haven't ever had a DOA on my moss...