Charging an extra fee at checkout

I have a question about the store check out process - if I charge a license, patent, type of fee for every purchase, is there something I can install at checkout so that the percentage is calculated and included in checkout?

Maybe a widgit or a code? That when someone puts a specific book in their cart, and then checks out, X% of that book cost is added to the overall charge

The checkout page can not be altered with code. It is an SS security feature.