Most airlines (all that I know of) do not have particular size or weight restrictions on the car seat or stroller you can check-in for your infant.

I was wondering if anyone had experience, or thoughts, on "cheating" with checked-in car seats/strollers. Here is our thinking: for protection, we'd put the car seat in its original box, and safety wrap the box at the airport. Inside the box there is plenty of "empty" space. Has anyone ever filled up that space with more clothing/useful items? What was the result?