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