If you would post the details of your flight, people here might have more useful suggestions.

China Southern is part of Skyteam, so I would have expected you could pick up flights with them on the Delta site. Alternatively, sometimes people book on a partner site - AirFrance, etc. In this case, one would try the China Southern site.

You didn't mention caring about frequent flyer miles, so perhaps that is not an issue?

I'm simply not aware of any issues with tickets purchased through Priceline so maybe you should just nail your good rate before it changes?

Romelle