Note: Not registered with USPS yet or don't know your developer credentials? Simply follow this quick guide from Shopify.
Adding your USPS credentials to Store Pickup + Delivery
Open Store Pickup + Delivery and head to the Rates Page. Scroll down the page to the Shipping section and select the Advanced Options
From the pop up modal, select USPS
Check Enable USPS rates and enter your Username.
Once entered, simply Save and your USPS rates will be active via the shipping channel of the app!