After Caroline departs, she receives an email that says thanks for staying and has the invoice PDF attached.

You can also ask 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 checked-out.

Get the invoice PDF - you can do that right after you generated the invoice, or also pick it up later. If you do it later, either remember the invoice ID or get it using the invoice API with filters. Or subscribe to invoice events using webhooks.

GET /finance/v1/invoices?reservationId={reservationId}
GET /finance/v1/invoices/{id}/pdf