(Bonus Tip: Use large, clear plastic recycling/lawn and leaf bags to put each car seat in. This keeps all the straps contained and ensures the bottom and back of the boosters do not separate. The airlines will want to put a luggage tag on each one, so fasten the straps and poke a small hole so the giant sticky tag can wrap around the chest clip. For seats that don't have straps (like boosters) turn them over and wrap the luggage tag around the string holding the cover on. Don't forget to bring new bags for the return trip; they likely will not be reusable.)
$4.00 for something you use for like two minutes.) It's a complete rip-off, but unless you can balance a 50-pound suitcase on your head, there's really no way around it.