When you're ready to make a payment online, you can enter a valid coupon code that entitles you to certain benefit.
This is how you can use the coupon code:
1) Click on the purchase button (eg. "SUBSCRIBE NOW"). If you have put a few products in the shopping cart, click on it.
2) Enter the code you're given earlier at the "COUPON CODE" box. Make sure it is NOT expired.
3) When done entering, click "CONTINUE" and complete the "PAYMENT" and "ORDER SUMMARY" steps.