Ok since I have tried this, shall comment on it.
I booked this
https://japanbusonline.com/en/CourseSearch/11500430001
From:Akihabara(Tokyo) To:Kawaguchiko(Yamanashi) |Highway Bus
Akihabara-Kawaguchiko Line
As the tokyo one was full, that was the next closest to my stop.
I missed the bus to Kawaguchiko due to overslept, though I still manage to get there by trains, it wasn't cheap.
Do note, on the train to kawaguchiko, don't get off at the Fujikyu Highland Station, it should be the stop after that. But if you do stop there accidentally, take a look at the crazy 3 way 360 rollercoaster ride.
On the way back, taking the bus to Akihabara from Kawaguchiko station was smooth, just show the driver your e-ticket will do. We were the only people on the bus on Kawaguchiko but seats are designated. The next few stop picked up much more people. The journey back was smooth. Nothing to complain. You will be dropped off at a bus parking place.
Overall, quite good. The bus is very on time and moves fast. In fact, it moved 1 minute early so make sure to be there 10-15 mins before actual timing.
Also, highly recommended to book the bus tickets in advance, I didn't see people boarding without tickets for mine. Not sure if they sell it on the spot. Referring to the buses back to the city, not the different coloured buses to tour the place. Those coloured buses ticket can be bought easily from my experience. I bought the 2 day pass though I only went for a day trip as it makes more sense.