When you click a reservation that is already paid, since version 5.19.12.01 the software will ask you what action you would like to perform.
By default the system will ask it. If you always want to open the reservation, you can change the setting “OFFICE”, “RESERVATION OPEN MODE” to “Reservation” or “Invoice”.
You can change this per computer and per application. That means Office programs can always try to open the invoice, while the booking might ask or open the reservation.