Method: Ensure coupon user usage limit is valid or throw exception.

Method: Ensure coupon date is valid or throw exception.

Method: Ensure coupon amount is valid or throw exception.

Method: Ensure coupon amount is valid or throw exception.

Method: Ensure coupon is valid for products in the list is valid or throw exception.

Method: Ensure coupon is valid for product categories in the list is valid or throw exception.

Method: Deal with remaining fractional discounts by splitting it over items until the amount is expired, discounting 1 cent at a time.

Method: Sort by price.

Method: Filter out all products which have been fully discounted to 0.

Method: Get items which the coupon should be applied to.