To more specifically answer OP’s question:



Delta - you can pay for checked bags anytime during the check-in window (T-24) either online/app or at airport kiosk; no price difference in paying online vs at airport



United - $5 less if checked bags are paid for before T-24; bag payment is an option once trip is ticketed. No price difference between online/at airport within T-24 window



American - $5 less for first (not second) checked bag if paid for online or on app (not at airport), payment only available during check in window (T-24)



Alaska - same as Delta



JetBlue - $10 less per bag if paid prior to check in (T-24). Can include it in the ticket purchase process, or add to reservation prior to T-24

*edit to note that these are for non-status/general status in regular old main cabin/economy