Most, if not all, airlines DO NOT charge for checking car seats. I put two Cosco convertible boosters in a large duffle 3 weeks ago, when I was flying on American and was not charged.
We also have a Duo Quattro (Graco). Just remember to take off the cupholders and stash them in your carry-on or suitcase before gate-checking. Those little buggers pop right off in the cargo hold and are over $8 each to replace. Happened to us last October when flying to WDW.