You can find your Promotion Coupons in the 'My Offers & rewards' section tab. The expiry will be mentioned on the coupon in Red. One the coupon expires, it will move to the Inactive tab.
The description & offer details will mention where the coupon can be used. You will have the option to deselect the coupon in case you do not want to apply it.
Whenever applicable, your Promotion Coupons will come pre-calculated with the original entry price struck out.