Its already been discussed that each of your customers are selfish and in that state, they are also self serving. Meaning, they may not want you to go out of business, but that doesn’t mean they won’t bend the rules to their favor.
The rule here is deal fairly with your customers, but don’t allow them to run all over you. If you provide additional service to your customer, make sure they know about it. Provide a Zero Dollar invoice detailing the work. If you have at strict policy (of any kind), stick to it. If one of your customers doesn’t like the work you performed, or you missed some expectation, try to make it right. There is not need to get angry or upset. If there’s a difference of opinion, deal with it quickly and efficiently. But with some customers, you could go the extra mile and they’d ask for two.
Whatever the cause, attempt to make the best of the situation, you may not end up with a great working relationship, but you may walk away with mutual respect.
Do the best you can do; meet your expectations and you’ll satisfy the majority of your customers.