First, to the best of my knowledge, NO seat belt company will recert anybody else's belts, only their own. So you will need to contact whoever's belts you have.
RCI used to offer a free recertification on their platinum belts, but they don't anymore. I don't even know if they still do recerts. Unfortunately I have a set of their platinums in a box that has never been opened.....and at least five years old. I guess they must have terribly deteriorated and become unsafe....while in the box.
