Karuizawa to Kusatsu Onsen Seibo express bus

I read that the luggage allowance on the express bus to Kusatsu Onsen from Karuizawa is only 10 kg . As our bag is about 20kg, how can we bring it along with us?
Also, is it possible to reserve tickets on this bus?

Consider using a delivery service:
https://www.japan-guide.com/e/e2279.html