Live Shipping Rates on Dynamics 365 ecommerce store front

[dexteraxelliant]

Hello All,
I have created an api and print labels everything in fulfilment solution, I have managed to call that api by creating a custom extension in dynamics 365 store front ecommerce , the problem is when it fetches rates, it shows but still shows free when i trying to upate that value it says,

"TypeName": "CartValidationException",

"Exception": "{\r\n  \"ValidationResults\": [\r\n    {\r\n      \"ErrorResourceId\": \"Microsoft_Dynamics_Commerce_Runtime_PropertyUpdateNotAllowed\",\r\n      \"LocalizedMessage\": \"You're trying to perform an operation that updates a read only property 'SHIPPINGCHARGEAMOUNT'. Either perform a different operation or contact your system administrator.\",\r\n      \"LocalizedMessageParameters\": [\r\n        \"SHIPPINGCHARGEAMOUNT\"\r\n      ],\r\n      \"ErrorContext\": \"Property SHIPPINGCHARGEAMOUNT is readonly.\",\r\n      \"MemberNames\": []\r\n    }\r\n  ],\r\n  \"ErrorResourceId\": \"Microsoft_Dynamics_Commerce_Runtime_AggregateValidationError\",\r\n  \"LocalizedMessage\": \"A validation error occurred.\"\r\n}",

how i can do this? i want to fetch a live rate and push it in cart and charge customer