Chicks, started chicks, and adult birds must be shipped in USPS approved boxes. Chicks can be shipped Priority Mail. All other lives must be shipped Express Mail. I put the link for the source of shipping boxes in a previous link. Here is my method for shipping lives that I posted on another thread.
I have a routine I go through when shipping lives, even baby chicks. I admit that I am on a first name basis with all my local postal employees, so that makes it easier. I ship a lot of live birds , and I always try to do so on Tuesday. On Monday I call my PO and give them a list of the zip codes I'm shipping to. They call me back on Monday and tell me whether it will be a 1 or 2 day delivery time for the given zip codes. I then notify the buyers when they should expect their chicks or adult birds. On Tuesday I take the birds to the PO about an hour before the truck picks them up. When I get home with my Express Mail reciepts I once again notify the buyers of the birds/chicks with USPS tracking number so they can track the shipment. Sounds like a lot of work, but having gone through the era when lives could not be shipped via USPS, and everything but chicks were shipped Air Freight or Rail, it's a peice of cake.