Sapporo-Noboribetsu Area Pass Reserve Seat ?

I'm Planning to buy the "Sapporo-Noboribetsu Area Pass" , I checked this pass is not include the reserved seat.

If i like to have the reserved seat for trip from "Sapporo" to "Noboribetsu"

I had Checked :

non-reserved seat : Basic fare: ¥2,420 Seat charge: ¥1,830
reserved seat : Basic fare: ¥2,420 Seat charge: ¥2,360

Am i need to pay the different ( ¥530 ) , or i need to pay full seat charge ( ¥2,360 ) ?

And how can I buy the reserved Seat ?