Oh I see, sorry for the misunderstanding. To go into more details, you need to send a LRAR, which is the only one with a legal proof and costs ~6-7€; otherwise they could pretend that you never sent it.

But my comment was on the fact that it was the only way to cancel. They could implement a system with single-click cancel (and it seems they did, here); it costs less than the current snail-mail system, and is more honest. But they don't because this system has a measurable impact on churn.