Originally Posted by
prister
As for United, there's another trick: if there's no economy seat at booking and you have to purchase econ plus at that moment. If that's the case, then the cpm might jump up to 5 or 6 after that, which isn't a good deal anymore.
I'm almost certain you can just not select a seat, and be assigned one at check-in. There's no requirement to purchase E+ on UA.