Okay, here's my experience of upgrading:
• Bought this app as part of an MU bundle. Had to recover my old key from the previous developer.
• Then upgraded via the site.
• Why the developer needs my telephone number is beyond me.
• New key not sent. I used key recovery to get it. Then the key arrived. After that my official email regarding my key arrived!
• Printopia is now an app rather than a preference pane. Not a good decision IMO.
• Printing images used to automatically open them 'like magic' in preview. It no longer seems to work. This feature has to be enabled by clicking on the printer, then its settings.
• Closing the window should close the app, given it is not required to run to Printopia to function.
• Overall, It would be better just to run the old Printopia until it no longer functions. I bought this so at least it would be supported beyond High Sierra. As mentioned, some documentation would be nice.
• The PDF and App workflows are really good - but should be more obvious to the end user.