I did a mod like this, but it was rather expensive.
I added a "Print Invoice" link on the orders menu that, when clicked, automatically sent the invoice to the printer, no confirmation boxes or anything. I had to use ScriptX from MeadCo (
http://www.meadroid.com/) which is $350 / year for 10 licenses. You can't buy it for any less than $350 -- I asked. They DO have a free version, but it only works from your local machine.
I'd be happy to answer any questions that you have. It was pretty easy to set up, though, with the downloadable examples they have.
--Kurt