To use your own carrier rates at checkout, you will need to activate the third-party carrier calculated (CCS) rates. If you are on the advanced plan, you should have CCS already activated. If you haven’t resolved this yet, you can use our recently launched app, JsRates. JsRates enables you to write your own shipping rules using JavaScript. You can use JsRates to connect to any third-party API endpoint (or your own carrier’s API) and display the rates at checkout. You can find more information on our website: JsRates: Custom JavaScript Shipping Rates for Shopify Stores, or additional information on our documentation website: JsRates Documentation: Your Guide to Custom JavaScript Shipping Rates. Or we can help you setup the shipping rates for you.