Depending on how you configured the "Always show coupon code box" setting for the Coupons add-on, the cart and checkout areas in the storefront will provide a coupon code box or a "Have a discount coupon?" link allowing buyers to access the coupon code box.
Coupon box in cart:
Coupon box on the checkout page:
Link in cart:
Link on the checkout page:
Buyers can redeem their coupon by entering their coupon code in the box and clicking Redeem:
If a valid coupon code has been entered, the coupon will be applied to the order.
If an invalid coupon code is entered (for example, if the coupon code is misspelled, expired, or otherwise does not meet the coupon application conditions), the page will inform the user of the problem.
For example, here is the result of using a misspelled coupon code: