Option to waive special registration fees

I charge a different registration fee for adult students than I do for children, and right now I just put it on each adult class, but there is no option to waive this for returning students or to keep it from stacking up if they register for more than one class. I currently have a code I send to returning adult students but not all of them understand how to use it, and if they register for more than one class they still get charged something because the code would only take off the registration fee for one class. If I check the box saying don’t charge registration to returning students, it needs to apply to all fees that are indicated as registration.