Hello sales16,
You can only copy an estimate to an invoice - the only workaround for this would be to create an invoice then mark it as paid straight away.
The copy function only copies the information into a new estimate, and so would not work in your situation.
Please use the feedback tool under the cog wheel within your account to submit this suggestion directly to the developers. They will then look into implementing this within future updates, if possible.
Thanks,
Talia