I sell software and have for many years, and I dont advertise it but I always give 100% money back to any unhappy customers (granted there is very few of them, for the last 2 years I have had one). Usually if someone mails me expressing unhappiness, I start by giving them a full refund even if they dont ask for it, then I mail them to tell them that I have refunded them, usually i let them keep the license, and then I try to engage with them to fix the problem.

Usually when this happens, its well worth the license fee to figure out whats wrong to improve the product, and the customer is eager to help.