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 need to 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. However, if you're on the Shopify plan or below, and you switch from monthly to yearly billing, you can ask to get the third-party calculated shipping rates feature added for free. For more information, contact Shopify Support.
Carrier-calculated shipping rates often show expected shipping times next to the shipping rate, however, they don't take into account your processing times. For more information on processing times, refer to Understanding and reducing processing time. For more information on letting your customers know about your processing times, refer to Addressing processing times.
If you use the Shopify Fulfillment Network, then refer to setting up your shipping rates for Shopify Fulfillment Network.