Hello,
FOSSBilling wasn't really built for email hosting specifically so there aren't controls created if you wanted to have an order form where someone enters an email address and it's automatically created.
You could use the hosting integration with a web-server control panel to sell "hosting" packages that only includes email and allow clients to manually login to the panel and create the email account, however this won't work if you intend to sell email hosting for domains you already own, as each client would need to have their own account and accounts can't share domains.
