Baggage fees are per bag and per direction. So one way = one charge. Round trip = two charges. If your flight has a connection, bags will generally transfer from flight to flight and no additional fees will apply.
With most airlines in the US, the overweight luggage fee kicks in if your bag is over 50 lbs, but Spirit Airlines adds an extra $25 when you hit only 40 lbs. ... You cant control keeping your weight under the limits unless you know whether that limit is 50 lbs, 40 lbs, 33 lbs, or some other amount.