I'm booking travel through my company's travel portal which requires me to book economy (taking the flight on either Delta or United). However, I would prefer to fly F for this trip. Is it possible to book the flight and then once it's booked, pay the difference out of pocket for a seat in F? And if not F, could I pay the difference for a Comfort+ seat? Most of the articles I found on Google discussed upgrading with miles rather than cash.

On my previous trips, I had to wait until I was airside and go to the customer service desk and pay to be put on the upgrade list for F when I would gladly pay up front to be confirmed in F