Shopify has a direct integration with USPS. Have you tried using this integration by putting your USPS account number? This makes sure that you do not have to copy paste information into Click-N-Ship. I am not sure Shopify will do an integration with click- N-Ship as it currently allows you to use USPS account without any problems. In addition, with an app that handles eVS account, like the Multi Carrier Shipping label app, you even bulk print labels without having to fulfil each one separately.