Ryanair already does this in the UK and Frontier does it in the US, so clearly someone has figured out the logistics of it. Both are ultra-low cost carriers (i

Ryanair already does this in the UK and Frontier does it in the US, so clearly someone has figured out the logistics of it. Both are ultra-low cost carriers (in the same model as Southwest used to be). As for payments slowing down boarding: I expect that it does, but the price info I see online suggests that the carryon fees are punitive (more than checked baggage, and with a 100% surcharge for paying at the gate). In other words, the purpose of the fee is more about discouraging people from bringing a carryon in the first place than the revenue it generates. Frontier doesn't seem to be shy about reminding customers about the gate pay surcharge, either.