If I may, before I address your question I would like to get some additional information and to make sure I understand what you are asking:
1) You currently have a process in place where an invoice document is e-mailed to a user or users and they review this from their Outlook Inbox - correct?
2) Is this "invoice document" an InfoPath form?
3) What is the current process? Meaning - what does the user currently do once they get an invoice in their Inbox?
I want to be sure of what you need before I offer any insight on what you are asking.
Scott Heim - Microsoft Office InfoPath and SharePoint Designer Online Community Support