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 display expected transit time next to the shipping rate at checkout, but they don't include your processing time. You can display delivery dates at checkout that include your processing time and transit time. To learn more, refer to Setting up delivery dates.
If you use the Shopify Fulfillment Network, then refer to setting up your shipping rates for Shopify Fulfillment Network.