The coupon code shows the discount amount on the checkout page before you submit payment. It shows the coupon entered in the cart, but doesn't calculate the savings until that later step. A number of users have used the Lite coupon successfully so you should not have problems.
You are right that an existing coupon entry should show you what it does before proceeding to checkout. I will make sure this gets fixed on Monday.
Again, this issue doesn't affect whether you get the 15% off. You do; as long as you entered the right code, and you are eligible, you will not be charged the full price.