In order to maintain and increase repeat business a company must deliver a good product or have a monopoly in the industry. In most cases it is nearly impossible to create a monopoly so the product better be good or the customer will let you and all of their friends and family know it fell short of expectations.
There are numerous referrals sites that give every disgruntled customer the opportunity to provide the details of your failure and urge others not to use your product. Should this situation occur it is critical that the customer receive a courteous and informed response. Every transaction will not be perfect but show that you care about your customer and will stand behind your product.