Who is responsible if Amazon.com package is not delivered?

I purchased a package on Amazon.com Merchant Seller. The item was promptly shipped via USPS Priority Mail with delivery confirmation and has never arrived. It has been sitting in my local post office for several days and is missing. Who is liable for this if I never received it? I have no verification of what was in the package since it never arrived. I wish sellers would stop using USPS. Most of us would pay extra for UPS or FedEx Ground.