The error code INSUFFICIENT_ACCESS_OR_READONLY is caused by the cart being no longer in a valid Checkout State during checkout flow customizations. A cart is an object that represents a collection of products and charges that a customer intends to purchase in the storefront. A cart has a Checkout State field that indicates the current state of the checkout process for the cart. The Checkout State can have values such as Draft, InProgress, Completed, or Cancelled. A cart can only be modified or updated when it is in Draft or InProgress state. A cart cannot be modified or updated when it is in Completed or Cancelled state. If an attempt is made to modify or update a cart that is in Completed or Cancelled state, an error with the code INSUFFICIENT_ACCESS_OR_READONLY will be thrown. This error means that the user does not have permission to edit or delete a record because it is read-only or locked. The storefront user does not have access to the Cart Delivery Method object is not a cause of this error code, as it is not related to the cart checkout state or data modification. The Cart Delivery Method object is an object that stores information about the delivery method selected for a cart in the storefront. An error has occurred during the cart shipping charge integration is not a cause of this error code either, as it is not related to the cart checkout state or data modification. The cart shipping charge integration is an integration that calculates and applies shipping charges to a cart based on various factors such as delivery method, location, weight, volume, etc. The storefront user does not have access to custom fields on the Order Delivery Method object is not a cause of this error code either, as it is not related to the cart checkout state or data modification. The Order Delivery Method object is an object that stores information about the delivery method selected for an order summary in the storefront. Salesforce Reference: B2B Commerce Developer Guide: Cart Object, [B2B Commerce Developer Guide: Cart Delivery Method Object], [B2B Commerce Developer Guide: Order Delivery Method Object], [Salesforce Help: Common Error Messages]

The root cause of why the CustomField__c field they added to the entity has no values under the header in the CSV file output is that the user does not have rights to the custom field. A user's access to a field is determined by their profile and permission sets, which define their field-level security settings. Field-level security settings control whether a user can see, edit, or delete the value for a particular field on an object. If a user does not have access to a field, they will not be able to view or modify its value in any interface, including Data Loader. Data Loader is a tool that allows users to import or export data between Salesforce and CSV files. When using Data Loader to export data from an org, Data Loader will only include fields that are accessible to the user based on their field-level security settings. If a user does not have access to a field, Data Loader will not include that field in the CSV file, neither as a column header nor as a value. The developer does not have the correct JDK that is recommended by Salesforce and this is known to cause issues with exporting custom attributes is not the root cause of why CustomField__c has no values under the header in the CSV file output, as it is not related to field access or Data Loader functionality. The developer does not have access to the object's metadata is not the root cause either, as it is not related to field access or Data Loader functionality. The object's metadata defines its structure and properties, such as fields, relationships, and layouts. The field is not populated is not the root cause either, as it contradicts the fact that CustomField__c has no column header in the CSV file output. If the field was not populated but accessible to the user, Data Loader would still include CustomField__c as a column header in the CSV file, but leave its values blank.
Salesforce References: Data Loader Guide: Export Data from Salesforce, [Data Loader Guide: Field Mapping], [Salesforce Help: Set Field-Level Security]

To enable the ability to add new shipping and billing addresses during checkout, a developer should enable Buyer Managed Contact Point Addresses within the Shipping Address standard component in the Checkout subflow. The Buyer Managed Contact Point Addresses is a feature that allows customers to add, edit, or delete their shipping and billing addresses during checkout. The developer can enable this feature by setting the buyerManagedContactPointAddressesEnabled attribute to true in the Shipping Address standard component in the Checkout subflow. The Shipping Address standard component is a component that displays and collects the shipping address information for the cart or order. The Checkout subflow is a subflow that defines the steps and components for the checkout process in the storefront. Creating a new shipping address checkout subflow that utilizes the Buyer Managed Contact Point Addresses component is not a valid way to enable this feature, as there is no such component as Buyer Managed Contact Point Addresses. Enabling Buyer Managed Contact Point Addresses within Commerce Administration is not a valid way either, as this feature is not configurable in Commerce Administration. Creating a Lightning web component that enables this functionality and replaces the current shipping address screen within the Checkout subflow is not a valid way either, as this is unnecessary and complicated when there is already a standard component that supports this feature. Salesforce Reference: [B2B Commerce Developer Guide: Buyer Managed Contact Point Addresses], [B2B Commerce Developer Guide: Shipping Address Component], [B2B Commerce Developer Guide: Checkout Subflow]

Explanation
Two advantages of using Lightning Data Service are that it combines and de-duplicates server calls and that it loads record data progressively. Lightning Data Service is a service that provides access to Salesforce data and metadata in Lightning web components. It optimizes performance and minimizes server round trips by caching data on the client side and sharing data across components. It also combines and de-duplicates server calls by batching requests for the same record or object and returning a single response. It also loads record data progressively by returning available cached data first and then fetching updated data from the server asynchronously. Communicating with other components and converting between different data formats are not advantages of using Lightning Data Service, as they are not related to its functionality or features. Salesforce References: Lightning Web Components Developer Guide: Lightning Data Service, Lightning Web Components Developer Guide: Work with Salesforce Data