If you've sent you customer a Quote and they've accepted, creating an invoice is done in no more than one click. Once you're in the specific job page you simply click "Create an Invoice" in the upper right box.
All the items you added from the Quote will automatically be transferred over to your invoice.
Similarly on the Dashboard one can create a Quote by adding items and either in draft or already sent to customer, it is easy to convert to an Invoice by simply clicking on "Create an Invoice"
and the Quote is copied directly to the Invoice where it can be edited or paid and sent directly.