Allow to add a max discount amount mainly used with discount percentage.

Problem

eg: Discount 50% with cap 20$.

1. If applied to an order with an amount of 100$
   calculated discount = 100 * 50 / 100 = 50$
   the discount 20$ ( capped )

2. If applied to an order with an amount of 30$
   calculated discount = 15 * 50 / 100 = 7.5$
   the discount is 7.5$

Rule

• If the discount is greater than the cap then we choose the cap amount
• If the discount is less than the cap, we choose the calculated discount.

We end up choosing the mix of the two.