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First, add the product(s) you want to your cart. When you are ready to purchase, click on 'My Cart' at the top of the page to go to your shopping cart.
Next enter the coupon code in the 'Discount Codes' box (see below), and then click 'Apply Coupon'. You should get a message saying that the coupon has been successfully applied, and a discount amount will appear below the Subtotal.