I'd really like it if Shopfiy offered this "free" direct shipping from the USPS. No middle man, on the spot payment for shipping, shipping label record, all directly from the Postal Service. Personally, find this to be a more useful tool, then any shipping app ever used. Also discounted postage rates, and shipping lable record comes along with shipping lable.
Below are links
Shopify does provide direct shipping from USPS. Since for the bulk of customers providing discounted rates is a must, hence Shopify USPS integration is preferred. And if you are planning to use your own account, then Shopify does have a way to use your own account as well. The General API developer guide that you are talking about is already being used in the integration or even with apps that take care of USPS shipping like the Multi Carrier Shipping label.
Am i missing something here?