Explanation
The best way for an administrator to quickly examine the relationships between several objects is to go to Schema Builder and check the boxes next to the object names. Schema Builder is a tool that allows administrators to view and modify the data model of their org in a graphical interface. Schema Builder shows the objects and fields in the org, as well as the relationships between them. The administrator can use Schema Builder to select the objects that they want to examine, and see how they are connected to each other. Verified References:https://help.salesforce.com/s/articleView?id=sf.schema_builder.htm&type=5

Explanation
Two likely causes that could make the second submission of the same JSON payload fail are:
The payment data must be updated. The payment data in the JSON payload contains information such as the payment method, amount, and authorization code. If the same payment data is used for the second order, it may cause a duplicate payment error or an invalid authorization error.
The Order Reference Number must be updated. The Order Reference Number is a unique identifier for each order that is generated by the external system and sent in the JSON payload. If the same Order Reference Number is used for the second order, it may cause a duplicate order error or an invalid order error.
Verified References:
https://help.salesforce.com/s/articleView?id=sf.order_management_order_api.htm&type=5https://help.salesforce

Explanation
Three steps that the administrator should take to implement this functionality are:
Create an External Data Source pointing to the external system's database. An External Data Source is a type of metadata that defines the connection and authentication information for an external system that stores data outside of Salesforce. The administrator can create an External Data Source using the Lightning Connect OData 2.0 or 4.0 adapters, which support read-only access to data in relational databases.
Create an External Object for each table of accounting data in the external system. An External Object is a type of custom object that maps to a table in an external system. The administrator can create an External Object for each table of accounting data that they want to access from Salesforce, and define its fields and relationships accordingly.
Create an Indirect Lookup field to connect to the Account. An Indirect Lookup field is a type of custom field that links a child External Object record to a parent standard or custom object record. The administrator can create an Indirect Lookup field on the External Object that references the Account object, and specify the external column name and value that match the Account ID.
Verified References: https://help.salesforce.com/s/articleView?id=sf.external_object_overview.htm&type=5
https://help.salesforce.com/s/articleView?id=sf.external_object_relationships.htm&type=5

Explanation
The correct time and way for creating an Invoice is at any point after the Fulfillment Order is created, by customizing flow "Create Invoice". This flow is a core action that creates an Invoicerecord from a Fulfillment Order record, and updates the status of both records accordingly. The administrator can customize this flow to suit their business needs, such as adding conditions, actions, or subflows. Verified References:https://help.salesforce.com/s/articleView?id=sf.flow_ref_elements_om_actions_create_invoice.htm&

Explanation
The best way for the administrator to meet this requirement is to modify the compact layout on the Credit Memo object. A Credit Memo is a record that represents a refund or credit issued to a customer for an order or part of an order. A Credit Memo has various fields and related lists that display information such as the credit amount, status, reason, etc. A compact layout is a type of layout that determines which fields appear in the highlights panel at the top of a record page in Lightning Experience or Salesforce mobile app. The administrator can use the Object Manager to modify the compact layout on the Credit Memo object and add the Total Amount field to the compact layout. This way, the sales team can see the Total Amount field in the highlights panel at the top of the Credit Memo record page. Verified References:
https://help.salesforce.com/s/articleView?id=sf.order_management_credit_memo.htm&type=5
https://help.salesforce.com/s/articleView?id=sf.compact_layouts_overview.htm&type=5