Discount from coupons is not visible to the payment page when customer pays

Hi, Payment integration has been set up and it works perfect. What I do not see is that when a customer is at my website and enters a discount coupon, he sees the reduction in the price immediately until the last page of my website. As soon as he is transfered to the payment provider website in order to type his credit card details, this discount is not visible anymore. What is visible is only the full price of the product. For instance, I sell 90 days subscription for 42 pounds. If I give him a coupon for 50% discount, I would like my customer to see in the payment page that "Today he will be charged 21 pounds, and 90 days later the charge will be 42 pounds" as all payment are recurring billing. The same needs to apply for the monthly subscription of course. What he sees today, is that his charge will be 21 pounds and as soon as he is transferred to the payment provider, he sees the whole price again of 42 pounds which is not correct of course. Therefore, any kind of discount coupons need to be reflected to the payment provider's page as well. Let me know if something is not clear. Thanks Giannis