Convert proposal to invoice

I have made an proposal for a test costumer. The testcostumer has accepted the proposal. (I wonder why I did not get a notification when costumer has accepted the proposal? I would expect to be notified!)
As admin I can see that costumer has accepted (when I look for it). So I have "done the work", and changed status to 'Completed'.
I was expecting that the proposal would be trasformed to an invoice. But no.
Will I have to create a invoice from scract, or can I use the proposal as a invoice-draft, converting to invoice upon completition?
When I finish an order, comming from a proposal, I expect to be able to send an email to costumer: "Work has been done, please pay attached invoice" (and a PDF is attached...)