I just looked at your settings and the problem is that you have your state abbreviation filled out in the postal code field of your ship-from address, when really that field should contain your zip code. I was under the impression that this kind of thing would throw up some kind of helpful error message, but I guess that isn’t the case. We’ll definitely be improving that.
In such cases, the complete transaction comprising of USPS request and USPS Response would be of definite help. Does Shopify provide that? We at Multi Carrier shipping label app do provide that although it would be useful at Shopify's level as most customers would not use apps.