WIller Bus Pass Reservations

Hey there!
I have a question about the bus pass. On the website I read that the bus pass costs 10000 yen for three days. I also read that you need to make reservations before you get on the bus. But I am confused now, does this mean, that I have to pay for the reservation again? I tried looking up if seats were available and saw prices up to additional 4000 yen. Or is this only if you purchase a ticket instead of a bus pass? Are there any additional reservation cost if I want to use the bus pass?
Thanks a lot for your help!

does this mean, that I have to pay for the reservation again?

No, you should make the reservation through the dedicated reservation engine for pass holders:
http://willerexpress.com/x/bus/dynamic/3/en/html/pc/buspass/index_ja.h...

Thanks! Since I do not own a bus pass yet, I cannot access the reservation system through bus pass. So no additional reservation fees? And also, how is availability for bus pass holders? How many days in advance do you think I should make a reservation? I will be travelling through April.
Thanks a lot again!

How many days in advance do you think I should make a reservation?

I recommend to make reservations as soon as you have fixed your travel plans. The earlier the better.

well, i do not know where I want to go or for how long I want to stay there. I wanted to plan like a week ahead. would you say then that the bus pass is no good option for me?

It is certainly possible. It comes with a certain risk that some of the buses might be booked out in which case you need to be flexible.