The postal service in the US is a public utility: the US Post Office is required by law to deliver to any US address. The fact that there are private companies delivering something called "mail" does not change that: none of them are permitted to deliver first class mail, that is a USPS monopoly, and none of them are required to deliver to all US addresses, but the USPS is.
I don't know why you would want Amazon to be a public utility.
I don't know why you would want Amazon to be a public utility.