Add new billing only user option
Add a billing administrative account, who deals with the subscription & payment, but who doesn't upload or download files.
Yes, the email address is a good idea, but we were also thinking along the lines of an admin who'd handle billing, new users, monitor usage, etc. so that the actual users wouldn't have to be concerned about anything except uploading and dealing with clients.
AdminSean Gay (Admin, Digital Pigeon) commented
We have revised the wording of this suggestion as we feel that aspects of it are worth considering.
Just to clarify, you'd like a new account type just for billing that doesn't effect your users quota?
We have a suggestion already to nominate an email address to send invoices (attached as a PDF) to, let me know if this would solve your issue and add some votes to it if it would.