You completely misunderstand the issue. You only get the customer's shipping address in Checkout. And the only way to inject an App to work with that data, is in Custom Carrier Shipping calls.
If you read my explanation I made ZERO mention of rates as being important. Strictly looking at address validation here.
What does your question We just need the Shopify interface built mean?
Your App use case is super limited here, and since you don't seem to like that, you need to be more open about what you really want to expose other possibilities if any.
