The additional 20% PRO discount is applied directly to the courses, you don't have to do anything!
As it is additional, you can use another promotion or discount code in your order. Then, in your shopping cart, you will see the final price of your order and the information on each discount in a detailed way. Remember that the discount percentage is not added, but rather the percentage discount on the price of the course is applied.
The PRO discount is only valid on individual courses that you buy to take from your profile, it will not be applied on Course Packs or Gifts.