Ok, the actual law:

You MUST LABEL YOUR PACKAGE. You must state clearly ON THE BOX that it contains live animals--you must write it big enough so everyone can read it. The point is, everyone handling the box who's paying any attention should know that there are live animals inside.

You must have the number and species, and common names (yes, all three items) listed clearly in a place that is accessible WITHOUT opening the box. It may be written on the box, or it may be on a slip of paper in an envelope attached to the box. If you hide it under the shipping label in the plastic pouch, that's FINE, but you must leave a note on the box (not hidden) stating that the information is there under the shipping label.

This is what is required by the Lacey Act, and there's no wiggle room. It doesn't matter if you use USPS, UPS, Fed Ex, or Delta Dash--you must still abide by this law, and label your package correctly. I will also point out that it doesn't matter how you feel, or whether you think it's a risk that people will abuse the package more because it's labeled, etc--this law is a LAW, and it is not negotiable. A person sending a package that is not labeled in this fashion is a felon. A person receiving a package that is incorrectly labeled is ALSO A FELON IF THEY KNOW WHAT IS IN THE PACKAGE. The only legal recourse for someone who has an incorrectly labeled package delivered is to refuse it, and of course no one will risk an animal's life that way. This is why it's so crucial for folks to demand that sellers come into compliance, if they aren't.

Now, because USPS does not permit the shipping of snakes, if you correctly label your package, they will reject it. Thus, the only way to ship snakes through USPS is to do so illegally--by not marking your package correctly. Is this more understandable for folks?

Now, as to the track records of UPS, Fed Ex, and USPS...
Get serious, folks--they all suck. Royally, repeatedly, and thoroughly. Arguing over which one is worse is pointless. You will find horror stories about all three services. People choose one--if they have good luck with it, they stick with it. If they don't, they switch to another one. Chances are, eventually they'll have a bad experience with that one too.

Now, individual hubs also vary in quality. In some places, you can't trust the local offices for one company, but another is fine. In a different place, it'll be the other way around.

In the past, I shipped geckos using USPS. They were fine, and on time. I've shipped animals using Fed Ex. Ditto.
I'm currently using UPS (through SYR). They've been fine. There was one delay due to weather, but that wasn't their fault, that was a snowstorm at an airport. It was only a half a day, and I had them hold the package at the hub when it arrived at the destination, and it was fine.

I'm quite sure that eventually, there will be a problem. It's pretty much inevitable. Shipping animals is risky, and we all know that. Which service the problem will be with, well, it could be any of them.