add extra field bonus for enrollment lower total

Please, how to add extra field in checkout, bonus for enrollment lower total. (Ubercart Userpoints)

for example.

Payment method
Select a payment method from the following options.
Order total preview:
Subtotal: 5,320.00
Order total: 5,320.00

Charge Bonus: *** points

Payment method:
Cash on delivery

I need to get this one ( Charge Bonus: *** points )

sorry for my english =)

Please help=)))