Breather or Breathing Bags are a somewhat new product made specially for shipping aquarium fish.
 

The Advantages Of Using Breather Bags

• The bags let oxygen in, and lets co2 out. Your fish will have an unlimited amount of oxygen.
• No air is needed in the bags, thus the use of smaller shipping containers is made possible.
• The absense of air reduces the sloshing during shipment making the trip less stressful for the fish.

Since I have started using breather bags I have had much better results shipping fish by mail. I hardily reccomend this great product.
The combination of these Breather bags and Bag Buddies have reduced my fish losses during shipping to less than 3%
 
The picture (right) shows a breather bag with a US quarter for size reference.
 
Please note that Breather Bags are not made for use with Bettas or any other air breathing fish. They need access to real air, not just dissolved oxygen. In addtion don't use Breather Bags for aquatic frogs, snails or any aquatic creature that needs direct access to air.
 
But for normal non labyrinth fish, Breather Bags are the best bags available for shipping.