Hello,
Of course, in order to process this kind of custom advanced html & php knowledge is required.
Now, to help you follow me step by step :
- In your file administrator/components/com_hikashop/classes/discount.php
- Go in this file to the recalculateDiscountValue function
- Find where the "$coupon->discount_flat_amount" is used to process calculation, and modify it like Nicolas guide you OR like you need to be processed.
Note : Take care to process your modification in each case in order to have a consistent results for each case, especially for product variants, and to keep this logical with your taxes settings.
Hope this will help you to get what you need.
Regards