Using registered post also ensures that they can't wiggle away by saying they never received it.

This is essentially required when dealing with French businesses in my experience. They absolutely will ignore anything that's not sent by registered post.

Mail them where? To do so, the company needs to provide an address where they would accept such correspondence and they may not do so.

You first ask them nicely where to send post and if they don’t give you one, you send it to the address where the legal entity is registered. In this case: 45, rue de Paris, 95 747 Roissy CDG cedex, France

Whether to accept complaints by post or not is not a company’s choice. Many countries have consumer protection laws that list non-exhaustive ways of making a complaint. When the postal means are listed there, refusing to review a mailed paper complaint could be a violation of that part of the law.