use Chained discounts
This is an additional option, only available for Fixed Price and Price Discount type Standard Price Lists.
This option gives you the ability to add an additional Line Discount to an order line that already contains a discounted price.
When defining a price list using the wizard:
On the Price Methods window, tick the Chained Discount option.
Enter the percentage to be apply as a Line Discount.
When a product from the price list is selected by a qualifying Customer:
If it is a Fixed Price item:
the price from the price list will be selected and printed on the order and invoice;
the chained discount percentage will be applied to the fixed price and applied as a line discount;
the final price charged will be the fixed price reduced by the chained discount percentage.
If the item has a Price Discount:
the selling price appropriate to the Customer will be selected and reduced by the price discount;
the discounted price will printed on the order and invoice;
the chained discount percentage will be applied to the discounted price and applied as a line discount;
the final price charged will be the discounted price reduced by the chained discount percentage.