It is rarely a good idea to rent a car seat/booster seat from a rental car company.
Just as easy to bring a low back booster with you on the airplane (it can be your child's carryon along w/ their personal item). A low back booster fits within the carryon requirements and fits easily in the overhead bin.
AND...if a child is young enough to require using a harnessed car seat, they should use one on the airplane as well so no need to rent one!
We have traveled repeatedly with both car seats (used on the plane) and booster seats (carry ons) and it really isn't a problem...just one more thing to carry.