hi,
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.
http://bus-en.fujikyu.co.jp/heritage-tour/detail/id/1/
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.
http://e.fujikyu-railway.jp/ticket/mtfuji_5th_station.php
Hope this help you.