Affiliates plugin, commission based on user roles

Hi,

How can the affiliates plugin define commission % based on WP user roles?
Example: Product Z = $100
Each affiliate gets different commissions for the same product.
User roles:
- Subscriber: 10% commission
- Contributor: 15%
- Author: 20%
- Editor: 25%

Not giving 25% to every affiliate who makes the sales. % given based on the roles.

Please advice.

Vi