To use a coupon, put your item(s) into the shopping cart, enter the coupon code in the [COUPON NUMBER] field and click [APPLY].
If you do not follow the correct steps (e.g. removing the coupon code/product from the shopping cart before the order is completed), your coupon will be considered an unused one. (Once the coupon expires, it will be invalid and can no longer be used.)
However, please note that once you cancel a placed order to which your coupon was applied, that coupon will be invalid and can no longer be used.