Allegiant and frontier would do.

Unlike hotels that have mandatory “resort fees” that there is no way to avoid and opt out of, airlines aren’t permitted to do that. If they call something a “fee” and split it out from the base fare, they have to give you a way to avoid it.

So airlines, clever as they are, invented a fee, often called a technology or convenience fee, that allows you to book from the comfort of your own home, in your pajamas. You can avoid it, by getting dressed, fighting traffic to get to the airport, paying for parking, and going to the ticket counter and buying the ticket…all within the 4 hour window per week the airline permits ticket sales.

Or you can just say “meh…not worth it” and pay the fee. The airlines are banking that you’ll do exactly that.

Also, the nice thing for the airlines is doing it this way means they can avoid the federal excise tax on the airfare, because that technically isn’t airfare. So it saves the airlines a ton of money in taxes.