Fix usps address verification bug, and origin address bug.

This resolves 2 pain points I encountered due to not using all configuration settings, but only wanting to use the minimal amount of config settings.  I.E. Just setting the api login & key.

1) Do not attempt address verification if the user is not configured to use usps account otherwise it results in a verification error breaking checkout.

2) Origin address should actually be determined by the stock location the order is shipping from rather than manually setting a origin configuration setting that needs to be json parsable...

Took me several hours to dig into and actually figure out what these errors were being caused by so definitely thing the readme could be updated with more detailed instructions, and at least this will prevent users from having to configure as much.