USPS is not going to give you the same level of service of FedEx.
It can be the cheapest service, but will also be the poorest performance.
FedEx has a far superior tracking system, being scanned at multiple points throughout the process. FedEx also provides a much more reliable Overnight delivery service. US Postal doesn't even guarantee overnight delivery to many addresses.
Using the ShipYourReptiles service, you can also insure for On-Time and Live Arrival for your live reptile shipments, no other shipping service offers that feature.
As for a driver leaving the package at the home/door without a signature, unless you specifically request a signature through Delivery Confirmation (which is a few dollars extra), a signature is not REQUIRED for delivery, regardless of weather.
If the driver encounters a live person at the address, they will get a signature, but unless Delivery Confirmation has been paid for, they can leave the package at the door "in a safe location" at the discretion of the driver.
It is especially important to make sure your recipient is anticipating the delivery of your package to avoid it sitting in the sun, or just on the porch, after delivery.