First of all, what are you shipping?
You mentioned USPS. Few things on that one.........first of all they do not hold shipments for 2 days. I have received lizards from them a few times and they came next day every time. As far as snakes it is against the law (unlike other companies where it is just against their regulations......big diff. between against law and against regulations) to ship snakes of any kind.
UPS has a policy, just like Airborne, FedEx, etc., where it is against their company regulations to ship snakes. On the other hand it happens quite often with ALOT of breeders/sellers.
Delta is the best way to do it. All you have to do is become a "verified shipper" or find someone who is one already. All they do is come to your house (or location you keep your snakes), inspect it for a minute or two, fill out some paperwork, have you sign it and that is that.
I personally use Delta for MOST shipments. The snakes arrive the same day, (when using Delta Dash and even more often than not will arrive same day when using priority or whatever they call it) can be insured and just alot more reliable than the above mentioned methods.
Hope it helps.
Brian Oakley