Explanation
Trigger invokes Queueable Apex method, with custom error handling process. Queueable Apex allows you to run asynchronous jobs that can make callouts to external web services. You can use custom error handling logic to handle any failures and retry the callouts if necessary. You can also use Database.Stateful interface to maintain state across transactions and ensure that each order is sent exactly once. This solution meets the requirements of sending order and line items directly to an existing finance application web service when an order is fulfilled, and ensuring that each order reaches the finance application exactly once for accurate invoicing. References: Certification - Integration Architect - Trailhead, [Queueable Apex], [Making a Web Service Callout from a Queueable Apex Job]