You can add custom shipping carrier accounts if your store is on the Advanced Shopify plan, is on the Shopify Plus plan, or has the carrier-calculated shipping feature. You can display your negotiated rates at checkout by using your custom credentials with Fedex, UPS, USPS, Stamps, & DHL.
First add your shipping address. To do this go to My Account > Addresses and add a shipping address.
To add a custom shipping carrier account go to Vendors Dashboard > Edit Account Details > Add Account details.