Hello,
If the orders have already been invoiced, you will have to commission your Salesperson through an AP voucher. However, if the orders have not been processed, you should be able to add the percentage to the customer default. but will probably also have to add the percentage to the order. You may have to tab through the line items on the order to make sure it "takes". If the quantity of line items is small enough, you may want to just re-enter them. I have found that although it advises to tab through all the lines when you change a price code, it doesn't always work (not sure if it would be the same for this scenario).
 
Hope this helps  
 
Holly