You can replicate existing invoices so that you don't always have to create a new invoice from scratch.
To do this go to the Invoicing menu, select the invoice you want from the list and click the Replicate button on top of the invoice.
This creates a replicate of the selected invoice with the default subject Copy of [the previous subject]. Recipient and other details come prefilled but remember to check the sum and due date!
Using the same procedure you can replicate existing or sent payments and online store products!