Coupon codes or merchandise credit codes can be added to the 'promo code' field at the cart, or in the checkout. Simply check the box 'I have a promo code' and enter the code into the field and click Apply.
Additionally, codes can be added in the Payment section of the Checkout. Simply check the box 'I have a product voucher, or promo code' and enter the code into the field and click Apply.
Please note that only one code can be applied per order.