If you have the box, you can put the booster back in the box and count the box as luggage. We've used boxes as luggage MANY times (we live in AZ, and all of our family is in NY, so we often have to bring boxes of Christmas presents back with us).
You aren't supposed to use a booster on an airplane. I did see someone fly with one and when I questioned her on it she was rather snipish, so I didn't press. This was flying back on New Year's Day, so I'm sure the flight attendants weren't looking too closely - they're were busy keeping track of the 12-15 unaccompanied minors flying with us.
~amey
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Mama to Daniel, 8 and Nicholas, 5 years old!
One of my most favorite places on Earth. Lake's Pond, Fort Ann, NY.