This one speaks for itself. Things like sticking to established policies comes to mind. An online contract is mutually agreed upon by both the provider and the consumer. Don't jeopardize client relationships by going against contract terms, and don't try to invent conditions later on down the line. If there isn't a written or verbal agreement on something, it's not enforceable.
Fewer surprises equals happier customers.
Keep your clients aware of everything your company experiences that may have an affect on them. But, although you can take precautions to try to prevent customer complaints, please also remember that some complaints will be unavoidable. Even so, just because they're unavoidable doesn't make them incurable. In most cases of dissatisfaction, your customer will ask for a refund. However, don't make the common mistake of thinking that refunding their money is the end of the problem.