Create an add-on feature that is based upon price point

I am working with a shopify theme and would like to make the following inclusion. I want to add an add-on feature that is based upon the pricing option the client has chosen. The add-on feature would only consist of 1 single item and the way it would work is if the client chose something between the $60-120 price bracket the add-on feature would cost $20, but if the price bracket was between $160-250 it would then change to $30, so it would be dependent on the price point the client choose. And this would have to then show up in the notification. I am not attaching any templates or anything, it will only be given when the job has been awarded