Yep, the tickets you're seeing now are what you'd get at the station on the day. They're a fixed rate and not linked to any particular service, so I'd recommend not buying in advance personally since there's no advantage to doing so.
It looks a better option for you would be to buy a DaySave, these give unlimited travel on any Southern train for £26. You can buy it now, but the only requirement is to purchase at least 3 days in advance unlike normal tickets
https://www.southernrailway.com/tickets/daysave