There is some highway bus + shibazakura combo ticket that can be purchase in advance. But I think tt is for traveling from shinjuku. Not sure if there is such tickets from Kawaguchiko.
Highway bus tickets itself can be bought online, so at least get tt one, since seats are limited. Park tickets at most just queue loh
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Yes, I plan to get the Shirakawago bus tickets, Kofu to Kawaguchiko and Kawaguchiko to Shinjuku/Tokyo Station. At least book online and reserve first. The combo that I read about is the bus from kawaguchiko station to the park return ticket and park entrance ticket, which is bought on the day itself.
By the way, I posted my full itinerary at the itinerary thread.