Create invoices in CiviCase
I have two comments:


CiviCRM is not really good in invoicing. So my advice would be to do the invoicing part in a seperate system, usually an accounting software might do the job
I have made an extension to link Contributions to a case, check it out at: https://github.com/CiviCooP/org.civicoop.casecontribution


May I ask what exactly you want to invoice from the case?