You can check a car seat or stroller at the counter without charge. You must have an agent check it for you though. If you add it to your bag count on the kiosk it will charge since it cant distinguish the difference between a bag or car seat. Check your bags and then when the agent comes to tag them tell them you also have a car seat you need to check if you want to get rid of it. Or you can take it to the gate to check and claim in the jetway at the destination.
Also, not sure if you're still Preferred, but Star Gold and higher get 3 bags to check free too. If you still are Star Gold, you can check it on the kiosk if it doesnt exceed your 3 bags and be done with it and not have to have an agent do it separate for you.