Yes, Quickbooks Online can be handy out on the field. But I regularly have two-three page invoices that probably should of started as an invoice. But we have "add-ons" that happen at the moment and often. Plus our part numbers we deal with are ten to fifteen digits. My minor dyslexia can cause havoc of course. 

We are wrapping up a job and someone wants an estimate for almost exactly the same thing. I groan under my breath because I know I can't give it it to him spur of the moment and in real life I have to make the estimate at home because its not a one minute effort as I have to print out the invoice or for me copy and paste part numbers into a new estimate. So between being exhausted at the end of the day, not wanting to pound on keys that night or have other things to do, like live a life, that potential customer may not see an estimate for 24 to 48 hours or more. And, I have entirely dropped the ball and forgot all about it until I find my note I made to myself days later. And don't say I'm a poor businessman, I'm mid 60s, worked hard all my life and have a thriving business. So yes, occasionally being to copy an invoice to an estimate for a new customer would be a serious time saver and a better customer relations tool.