Koala Flex is available on most ferry routes and can be added to your booking in the checkout before you pay for your booking. If Koala Flex is available for the route you're booking, you'll be able to select it at the bottom of the Customer details page by clicking the radio button in the top right of the Koala Flex panel.
If Koala Flex is not available for the specific route your booking, the Koala Flex panel will not show up on the customer details page.