In case you update your shipping settings in shopify follow the steps below to update it to your Merchant Center.
Shipping settings are updated only once using AdNabu, during the onboarding process.
In case the settings have been changed, you can go to Merchant Center and delete the previous settings.
Then you need to come back to Google Shopping Feed App and click on the "copy shipping settings" button.
This will ensure the updated shipping is copied to your Merchant Center.