I thought I replied to this?? Must have been another USPS post.
First I will preface this with - I used to think USPS shipping for finches was literally a joke an older hobbyist told me to make me laugh. I thought it sounded archaic and dangerous. Then I actually tried it. Now I've shipped USPS dozens of times in the past couple years and I would never do it differently.
YES it is against USPS regulations to ship finches. I have heard of people who had their finches turned around because someone somewhere knew what they were and that they were against regulations. I heard of that happening once through hearsay out of the hundreds of other USPS finch shippers I spoke with.
That being said, they will openly accept finches. My local post office accepts finches as if I had told them I was shipping a box of rocks. They don't care. BUT they WILL stop me if I openly admit to putting water in the box. They also now have the "no water" regulation printed out and posted at all counters in their main office. Sorry about that, it was definitely my fault.
I personally use their pickup option so I don't have to take a cab to the post office any more, that was such a nightmare of a trip every time. Something would go wrong and I'd end up having to go home and try again a week later. They are much more strict at the post office than they are if you schedule a pickup from your house.
THEIR SYSTEM IS NOT PERFECT. As Bob and others have pointed out, weather is definitely a factor. That's why I have strict regulations when it comes to weather - wind, heat and cold at both the origin and destination. They DO have "animal trained" staff who handle the live shipments in transit, they DO use temp controlled environments, etc. Does that mean everyone who comes into contact with the live bird shipment will be specially trained? No, of course not. Does that mean they are in perfect environments the entire trip? No, of course not. These are things you need to be prepared for if you're going to use USPS shipping. INCLUDING delays. Across country sometimes it can take up to 4-5 days. Also why I ONLY ship on Mondays to avoid the birds getting stuck in the post office over the weekend.
THIS INCLUDES LOST SHIPMENTS. How do you prepare against such things? Well, how do you guarantee the best service with anything in life? Pay a little extra for a special service. As someone else already suggested, they do have special handling options. OR you can do what I do and insure the shipment for $500-$1000 for only $5-$10. I do this out of my pocket every time as a courtesy. If you buy insurance, it shows up on the label. I have NEVER had a shipment go missing as a result. Do they pay out for live birds? No, NEVER. It is in their regulations that they will not, even if you pay for the insurance online. But will that stop some entry level warehouse working employee when they read the label and ponder stealing the box for themselves? YES. I can attest to that, else I would have probably had a handful go missing by now.
The USPS staff knows their regulations against finches are outdated. I've spoken with them at length and they claim they've been working on getting it changed for years now. Post offices will still accept it because hey, it's money and people do it openly all over the country. There are multiple large scale brokers/flippers who have USPS Express listed as a shipping option on their site. I don't think that will EVER happen and the regulations will always be there. The entire reason they don't allow the birds through their system is because they prohibit water in all live shipments. The reason for that is pretty obvious. Water + cardboard boxes = disaster.
There are obviously safe ways to do USPS shipping, since so many of us do it regularly and successfully. I've shipped to every corner of the country without any hiccups. Again, always insure the package. Always make sure the substrate is thick and water proof. Put PLENTY of food and water in case of delays - I usually put 1-2 weeks worth in their with them depending on the length of their trip (2 or 3 days and I give them a week, 4 or 5 days and I give them 2 weeks). This does waste a ton of seed but it gives me peace of mind. Zip tie drinkers and make sure they are 1000% secured. Zip tie millet. Sponges in the drinker bottoms to prevent spillage (I use all natural fiber sponges which work aces).
AIRLINE SHIPPING and USPS EXPRESS are the EXACT SAME THING!! They both use airline and ground transportation to get them from point a to b. Yes, USPS uses more ground transportation and does not always guarantee next day delivery but it's much more cost effective. Both shipments sit in the bellies of planes, both go through the exact same process. Do airlines make sure your birds are in the perfect environment the whole time? Nope, it's just like USPS - you get what you get and need to plan around weather to be safe. The ONLY other difference is that USPS will deliver the birds to your door or local PO. (They call as a courtesy but you can ask them to bring the birds to your door and they will.)
Without USPS shipping imho I don't think we would have nearly as many hobbyists, breeders, or even enthusiasts.