I second this!
The people who use Billings are the kind of creative types who can sometimes make mistakes that are only spotted by the client. And for the record, issuing credit notes is NOT AN ANSWER, and I speak as one who married an accountant. I asked about this a while ago and the reply was 'if you could alter invoices, it would make Billings a dodgy app' (I paraphrase), but I do note there is a way to go back to the original time slips now.
That doesn't help us 'itchy trigger/invoice finger' types as I think the edited invoice goes out under a different number, which makes reconciliation a bummer.
I think the 'Issue Invoice' needs an undo function.
After all, one client of mine sometimes requires an invoice address in another country - I don't learn about this until AFTER the job is done, and usually AFTER the invoice is issued. Okay, so I invoice to the UK, I get an email back saying 'oh, sorry, this is an EU job, not a UK job, can you re-issue?" so I do that, get an email back saying 'my mistake - it's UK after all', so I go back to Address Book, unpaste the UK info from the Notes over the top of the recently pasted German info, and try to re-issue under the same invoice number, then get the call that it's an EU job after all. Cue opening of 10 year old Scotch.
So I could have credit notes going back 4 iterations, and change my Master Address Book info for my client between UK and German addresses, whilst emptying a bottle of aforementioned rather nice Scotch. Or I could save sanity, liver damage and bandwidth by being able to ALTER INVOICES I HAVE BEEN FOOL ENOUGH TO ISSUE BEFORE THE CLIENT AGREES TO IT!!
But the nurse says I must rest now.