Yes, that's the correct rate (should be valid as an off-peak return - check the timings that this allows, though). Southeastern is the best online place to buy this, but the only advantage to buying in advance is not having to figure out the tickets machines or wait at the ticket counter - the fares are walk-up, and the trains are unreserved. You do all have to travel together in both directions.

Do also check the station parking rates.