Max provision and %

Hi!
Great theme, fast and looking good with one of yours child theme. But we do miss one thing. We would like to have an option for setting the provision for a max amount. Lets say we have a 8% provision on sales, we would also like to have a max provision together with the % . Our sellers are offering some quite expensive products, so 8% goes well up to around $500. Over that amount we would like the provision to be fixed, lets say $40. Is it anything you have considered, or can you or someone you know code this function.

THX for a great theme, and keep up the good work!