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 Reference: https://help.salesforce.com/s/articleView?id=sf.order_management_order_api.htm&type=5 https://help.salesforce.com/s/articleView?id=sf.order_management_payment_api.htm&type=5

Explanation
Use Data Loader. Data Loader is a client application that allows administrators to bulk import or export data from Salesforce using CSV files. The administrator can use Data Loader to import data into Order Management objects, such as Order Summary, Order Item Summary, Fulfillment Order, etc., or export data from these objects for backup or analysis purposes.
Verified References: https://appexchange.salesforce.com/
https://help.salesforce.com/s/articleView?id=sf.data_loader.htm&type=5

Explanation
The object that the Ensure Funds Apex Flow action looks for when resolving Invoice balances associated with an Order Summary is Order Payment Summaries. This object represents the payments made for an order, and it has a lookup relationship to both Invoice and Order Summary objects. The Ensure Funds Apex Flow action uses this object to calculate the balance due for each Invoice and update its status accordingly. Verified References:
https://help.salesforce.com/s/articleView?id=sf.order_management_order_payment_summary.htm&type=5https:

The objects that will be created in Order Management when a customer orders a product through B2C Commerce but changes the quantity ordered after the Order Summary record is created in Order Management, and then returns one of the products are:
A Return Order object. A Return Order is a record that represents a return request for an order or part of an order. A Return Order has a lookup relationship to both Order Summary and Change Order objects, and it contains information such as the return reason, status, date, etc.
A Change Order object. A Change Order is a record that represents a change request for an order or part of an order. A Change Order has a lookup relationship to the Order Summary object, and it contains information such as the change type, status, date, etc.
Verified Reference: https://help.salesforce.com/s/articleView?id=sf.order_management_return_order.htm&type=5 https://help.salesforce.com/s/articleView?id=sf.order_management_change_order.htm&type=5