If you are going to do free shipping you can change the shipping options by hitting the edit button at the top of this page. South park seasons with free shipping usually bring 5000+ credits. :)
You would have still done well on credits with a shipping fee just so you know. :)) i sold season 4, 5, and 6 seperately (used) with a shipping charge and got about seven thousand credits as my lowest ending bid. Just trust your own instincts!