After Caroline departs, she receives an email that says thanks for staying and has the invoice PDF attached.
You can also ask for her feedback about her stay.
To get the reservations of the guests who have checked out, use webhooks to receive a notification when a reservation changes to
Get the invoice PDF - you can do that right after you generate the invoice or pick it up later. If you do it later, remember the invoice ID or use the invoice API with filters. Or subscribe to invoice events using webhooks.