Caveat: On Spirit you can pay for the bags as soon as you buy the ticket-- you don't have to wait for the 24-hour mark. I don't know of any other airlines who do things that way, though.
Yes that is true I just booked my United flight and you pay when you check in and you can pay for the round trip all at once too. This saves you the hassel when you are checking out at the hotel for your flight home with DME.