From Nielson Norman Group: https://www.nngroup.com/articles/applying-discounts/
Summary: Coupons and other discounts should be easy to apply and shopping carts should clearly display how the total was affected by the promotion.
Many ecommerce websites offer a variety of sales, coupons, or discounts to their customers. In our research for the new fourth edition of the Ecommerce User Experience report series , we evaluated users’ experience with many of these promotions and discounts across a variety of websites.
Shoppers love deals, whether in the form of site-wide sales, select sale items, or coupons. However, for users to be successful using promotions, a website must do two things well: (1) it must communicate these various promotions effectively throughout the site, and (2) it must also make it easy for users to get the deals being communicated. In this article, we will provide a set of guidelines for helping users take advantage of the promotions and apply them to their purchases.
Applying Discounts and Promotions
When it comes to coupons, the easier to apply the better. Not only does this approach make customers happy, but it also prevents them from being disrupted or unnecessarily distracted during checkout.