Third-party carrier-calculated shipping rates
If you have your own account with a shipping carrier, then you can display your negotiated shipping rates from that carrier to customers at checkout.
You must have the carrier-calculated shipping feature on your store's Shopify subscription plan to connect your own shipping carrier accounts to Shopify. This feature is included in the Advanced Shopify and Shopify Plus plans, and can be added to any plan for a monthly fee. For more information, contact Shopify Support.
If you use the Shopify Fulfillment Network, then refer to setting up your shipping rates for Shopify Fulfillment Network.