First, use the link previously posted to get yourself some AirTran $25 coupons. You will need to send it to two different e-mail addresses for each round trip ticket. This way, you can book your round-trip as two one-ways and save a total of $50. You can still book it as a round-trip, but then you can only use one coupon.
Second, when you are ready to book, go to kayak.com. There, you enter your dates of travel and they will list available flights and airlines. Select the AirTran flight that you want and it will send you to the AirTran website. Now, when you book your flight, you will be able to pre-select your seats and you will not be charged the $6 fee.
If you do these two things, your $79 flight segment will only end up costing you $69 which will
include your seat assignments and a checked bag.($79 - $25 coupon + $15 for a checked bag

)
I did this for my trip in October. Our DTW to MCO leg was listed at $54 and our return was listed at $44. I booked them separately using the above steps and paid a total of $69.20 for a non-stop round trip flight. I also have A+ credits that I've redeemed for upgrade vouchers so now I will be flying business class which includes 2 checked bags for free. AirTran Rocks!