Have a read of this previous post:
https://www.wix.com/corvid/forum/community-discussion/canada-post-integration-to-wix-stores
https://www.canadapost.ca/cpo/mc/business/productsservices/developers/tour/forecommerce.jsf
https://www.canadapost.ca/cpo/mc/business/productsservices/developers/services/gettingstarted.jsf
https://www.canadapost.ca/cpo/mc/business/productsservices/developers/services/rating/getrates/default.jsf
https://support.wix.com/en/article/setting-up-tax-in-wix-stores
https://support.wix.com/en/article/setting-how-shipping-and-tax-displays-in-the-wix-stores-shopping-cart-page
https://support.wix.com/en/article/including-shipping-fees-when-calculating-tax-in-wix-stores
Plus, you might want to have a read of these pages too about Wix Fetch and HTTP Functions:
https://support.wix.com/en/article/corvid-accessing-third-party-services-with-the-fetch-api
https://www.wix.com/corvid/reference/wix-fetch.html
https://www.wix.com/corvid/reference/wix-http-functions.html