Currently there is a very large market for additions to child safety seats, all touted as following regulations and being safe to use. However child safety seat manufacturers specifically state in their manuals that anything that did NOT come with the seat, is not safe, as it has not been tested and WILL void the warranty.
The marketing of these products, as labelled is putting children at risk and should be looked into.
The types of additions that are up for discussion include: buntings, hanging toys, seat belt adjuster/tighteners, car seat protectors, shoulder belt paddings, etc.