The earlier you buy your bags, the less they will cost. If you're purchasing bags more than 24 hours before departure, you can add bags on Spirit's website at the "My Trips" tab. If you're purchasing bags fewer than 24 hours before departure, go to the "Online Check-in" tab instead and purchase bags there; this will cost more than had you purchased the bags earlier. You can also add bags at the airport, but this will be the most expensive option since the process requires an agent and slows down check-in.