Maybe I'm missing something but what's the point of the Invoices module? Trying it out, it doesn't seem to actually generate an invoice so seems like you still need a separate system to do that. So aside from just tracking invoices that you
make in another system and creating double the work, what's the actual purpose for it? Is there no way to generate an invoice the way you would with a Quote like with Mail Merge?
There are several options and methods, depending upon needs and skill level. Invoices in CRM can be used also to send hard copy invoices to customer. One would have to create a custom report (typically in SSRS) and then generate it from CRM in order to send
to a customer.