When we pay OOP we mostly fly JetBlue. They don't charge for seat selection and, according to them, they don't oversell their flights. A few years ago they added a tier system so you can book a basic flight with no luggage, a seat with one bag or a seat with two bags. Depending on what you choose you can pay more or less. It's still a lot cheaper than other options but who knows, they could start charging for seats at any time.