After receiving several plants from the generosity of TF members, I think someone with more experience mailing plants should write a protocol on this subject. Please do not think that I am criticising TF members; USPS is partly to blame too. I am new here and I have not mailed out any plants myself, as I am just building up my collection.
Looking from the receiving end, this is what I can think of. The main problem is in the packing:
1) Make sure the contents are fully cushioned from being thrown about like a rag doll in the empty space of the box using crumpled newspapers, paper towel, foam peanuts, LFS, etc.
2) Make sure there is enough moisture to sustain plants for the duration of the journey.
3) Seal plants in ziplocks and if possible take it out of the pot with just the rootball and wrap it with a damp paper towel and place it in a ziplock. This will save on postage without the extra weight of the pot and extra wet soil. Ziplocks also prevent moisture loss during transportation.
4) If necessary, mail it in an insulated box to protect either from the cold or heat.
I am sure other members have some more to add to mine. Thanks to you all for your generosity, and what a great forum. Have a good weekend.