You handle that however you want. We give you space - you create and price your own packages, handle your own billing, and so on.
If it is a server issue we would sort it out, just like with any other account. Ideally you would be able to handle the basic account-level stuff, but of course we're always here to help. Regardless of the issue though your customers should be contacting you as their provider. If you are unable to help them, you can contact us. Please do not send your customers to our help desk.
They can purchase hosting plans from us and we will take care of them.
It's all yours, so if you don't brand it, nobody else will.