In your checkout-level Purchase Automation settings, click Add Action, and you will see an event for Checkout Flow Complete.
This event is triggered when the checkout flow is fully completed and all upsells in the flow have been accepted or declined.
There may be a 2-hour delay or grace period after the flow is completed until this automation runs, to give customers time to accept or decline all upsells. If there are no upsells in the checkout flow, this automation will run immediately.