I have found in the past that the public service lines are a good earner while I build my company, but I usually get to the point where I cancel the public service lines and build my own commercial lines along the same (or similar) routes.

I don't believe there's anything special about the POI's along the public service lines, and my experience is that the lines are just as well travelled when I turn them into commercial lines.

I'm not aware of any way to negotiate higher payments, nor any way to easily transfer the line to commercial. But I do a couple of screenshots of maps and stops, and rebuild the line manually.

Cheers,
Chris.