We check our car seat in a generic brand version of this.

I often put extra stuff in there. Usually just a bag with blankets or dirty clothes and the purpose is more for extra protection of the car seat vs. cheating the weight rules. But there have been trips where we took shoes, diapers/wipes, or other heavier items from other bags and put them in there to get those bags under the weight limits. Usually the car seat plus whatever I add is still under 50 lbs by a lot; no one has ever checked what it inside.