Have a short trip booked in August for BLI-RNO. Didn't pay for checked bag online as of yet, as I am not sure if I need to. I have plans to pack a few shirts and other unmentionables in a back pack which will fit under the seat in front of me without issue. The length dimension of my backpack are slightly larger than the allowed measurements. However, it will be less than half full and can be folded over to fit those dimensions easily. How do they measure? I don't want to have to pay in advance if I won't need to, but I also don't want to be stuck with a larger bill if they get sticky about it. Anyone flown them recently had an issue with this?