Honestly, I'm a prime member and I'm not sure it's fair to prioritize shipping prime members orders over anyone else's.
As long as they guarantee that I get my order based on in stock quantities when I submitted, then that's good enough.
If they want to give prime members an added bonus over non-prime, I think it would be more fair to just offer certain hard to get or low stock items to prime members only.
Either mark them as "Prime Only", put them in a Prime Only Section, or only have them visible to prime members when they log on.