"News of bad customer service reaches more than twice as many ears as praise for a good service experience." - White House Office of Consumer Affairs
Nothing is more powerful than a trusted friend telling you about a service or product. Word of mouth has been and always will be the advertising method with the highest conversion rate. We know this about sales referrals, but we tend to neglect the powerful effects of negative word-of-mouth advertising. Although it's difficult to quantify, it definitely hurts sales, especially if a bad experience goes viral.
Treat your customers well and keep records of your communications, so that you'll have evidence to prevent brand damage if someone decides to try to extort you using lies.