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 update that value it says,

“TypeName”: “CartValidationException”, cannot update a readonly value, i read Microsoft documentation, it says i need to create an extension for over-riding Commerce/$metadata#Collection(Microsoft.Dynamics.Commerce.Runtime.DataModel.DeliveryOption)”
i am clearly clueless how i can do that?

We have called this api from store commerce frontend, but cannot update value