if you only travel in kawaguchiko area(three lakes), you can buy a sightseeing bus pass. Here attach you the information, you can choose anyone that fit your trip.
And if you want to travel from kawaguchiko to Mt. fuji 5th station, you need to buy a normal ticket. I don't think there is any cheap pass you can use between kawaguchiko station to mt. fuji 5th station. The price is 1540JPY(one way) or 2100JPY(round way).
Or there is a pass valid for two days from Otsuki station to the Mount Fuji 5 th Station and you can get off any station you like. However as you already book a bus ticket, I dont think you will need this. Any way, I will also attach you the link.
Hope this help you.