Coupons will only be applied to the amount that is immediately charged at the time of checking out as indicated by the 'Total Price'. So, if the product being purchased has a trial then the coupon will be applied to the trial. 

Essentially you have two options when creating a coupon:

  1. Apply the coupon to the first charge. In the case of a product with a trial, the discount will be applied to the trial.
  2. Apply the coupon to all charges and the discount will be applied to the subscription each month (in addition to the trial).

The checkout page will only show the discount on the amount that it is immediately being charged, so that would be the trial amount. So, for example, if your product has a free trial, then there is no immediate charge when checking out, so the total will be zero and there is nothing to discount. If you've created a coupon that applies to 'all charges', after checking out, the member can go to their MyAccount page > Subscription section, to verify the discounted price for the subscription per month.