Here is my final say on it.
Prorate it, simple and works for both parties.
Gives them a peace of mind that the server won't go into flames. Given that they can just fire their provider at any time and anywhere.
Gives you a peace of mind that for every day that you provides a working service and ensure that the customers are happy you got a pay day. Failing that you only lost what you didn't even delivered yet.
Fair for both parties and even gives the customer a slight "edge" given the nature of such a policy.