You can modify the registry entries to have the print driver called "Postal Shout Printer" -- upon user selection, it prints the document to your desired format and then launches your executable. From this, you get the filename and start all the processing. Step One: Who would you like to send this to, Step Two: starts the upload, etc... I built this wizard functionality over 10 years ago for a client and that's what triggered my idea that you've started to execute on. Back then, it was VB6 exe that was being launched and it was incredibly stable. I wrote some winapi hooks to get byte transfer speeds and calculate remaining upload time, etc... Also, hooked it into a CRM system so that the document could be linked to the CRM. Slightly different use case, but same net result.
I'm sure that driver isn't the only one capable of launching your own executable. I did a lot of research back then (including writing my own driver) and had many options - the pool of options must be much greater now.
I had the ability to create everything on my own back in the day, but I didn't have the capital to purchase the equipment necessary to make it all happen. That would have ran into the millions of dollars and I was never able to push it through (didn't really try that hard I guess). I should have just done the printing manually to build up a user base - but I didn't know much about business or even how to do it then. Heck, I still don't! :-)
My email is in my profile if you want more info. Or, feel free to ask here - your choice.
