Helen here from Shopify Support. I can help you with adding the Third- Party Carrier Calculated Shipping feature to your account!
There are a few ways to add it on:
Once you’ve decided which way you would like to include CCS in your plan, Shopify Support will need to enable it for you. Please reach out to live support for immediate assistance with activating it. Or else feel free to reply to the email I have sent you, and I’m happy to switch it on for you as well.