Percentage based Line Items

The lack of ability to add line items that are percentage based is annoying. I have multiple items I need to manually calculate that is percentage based and not being able to have it is annoying. We have a line item for discounts so adding a line item for additional percentage or dollar based charges shouldn't be difficult. But for example I offer 1 Year warranty for 10% of the purchase price or 3 Year warranty for 15% of the purchase price but I have to calculate this instead simply adding a line item.