I know this is pretty complex task, I don't expect a solution, but a guide on where to start.
(Drupal 7 is in question)
I need to apply X% discount for every Nth products that are in the same category. Category, X and N needs to be imputed by user.
So far, I was able to make a rule (with fixed X, N and category) that calculates a needed total discount that needs to be subtracted from total amount. How can I apply that number (or should I do something different?), and how to get user's input for X, N and category?
Thank you for any help.