I pay for checked baggage on line for Airtran all the time. You can do it when you check in for your flight. You can also print your boarding pass then, or can have it printed later. We found out this last trip that if you are at a Disney resort, and flying out of MCO, you can check in your luggage at the resort,even if you are not using Magical Express for transportation to the airport. We left off our checked bags the morning of our last day, and then only had our carry on bags to deal with at the airport. We got our boarding passes at the resort, and were able to go straight through security after returning our rental car, just as if we had done curbside check in. But, you do need to pay for the baggage fees on line in advance.