If you have a promotional voucher/coupon which entitles you to discount off your order, you can enter this on the shopping cart page.
Enter the code of the voucher in the "Voucher code" box and click "Update" to ensure it has been applied to your order. Any discount that is applicable is applied to your order and show as a discount.
Please check the conditions of your voucher if it does not appear to work on your order as some have special conditions.