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

The best way for the administrator to accomplish this task is to add the "Order Routing Rank By Closest Distance" action to the fulfillment flow. This action is a flow core action that ranks the fulfillment locations by their distance from the delivery location, and returns a list of ranked locations. The administrator can use this action to find the optimal fulfillment location for each order based on proximity. Verified Reference: https://help.salesforce.com/s/articleView?id=sf.flow_ref_elements_om_actions_order_routing_rank_by_closest_distance.htm&type=5

Two statements about the Order Summary object that are accurate are:
It does not exist without the original Order object. An Order Summary is a record that represents the financial summary of an order that is received from an external system, such as B2C Commerce or B2B Commerce. An Order Summary has a master-detail relationship to the Order object, which means that it cannot exist without its parent Order record.
It is read-only for financial order data. An Order Summary contains financial order data, such as total amount, tax amount, discount amount, etc., that are populated from the external system when the order is created. These fields are read-only and cannot be edited in Order Management.
Verified Reference: https://help.salesforce.com/s/articleView?id=sf.order_management_order_summary.htm&type=5

The Order Delivery Method and Delivery Method objects are used to configure delivery methods for orders. A delivery method is a way of delivering an order to a customer, such as standard shipping, express shipping, or in-store pickup. A Delivery Method record defines the name, description, type, and cost of a delivery method. An Order Delivery Method record associates an order with a delivery method and stores additional information such as the delivery address, tracking number, and status. Reference: Order Management Objects, [Delivery Methods]

The type of relationship that exists between FulfillmentOrder and Location is Many-to-One Lookup (Location) FulfilledFrom Location Id. A Lookup field is a type of custom field that creates a relationship between two objects, but does not support rollup summary fields. A Many-to-One relationship means that many child records can be related to one parent record, but not vice versa. In this case, a FulfillmentOrder has a Lookup field named FulfilledFrom Location Id that references a Location record. A Location is a record that represents a physical location from which order product items are fulfilled, such as warehouses and retail stores. A Location can have many FulfillmentOrders associated with it, but a FulfillmentOrder can only have one Location. Verified Reference: https://help.salesforce.com/s/articleView?id=sf.custom_field_types.htm&type=5 https://help.salesforce.com/s/articleView?id=sf.order_management_location.htm&type=5