Returning BRP after Extension

A couple of questions regarding expired BRP.

1. I've recently had my spouse visa extended but do I still need to return my old expired BRP card or is this only for those that apply for British Citizenship?
2. Has anyone ever reported being fined for not returning their old BRP?
3. Is the address to send a BRP to free or does the envelope have to be stamped, if sending in the UK?

You can either retain the old voided BRP card with you as souvenir or return it to
BRP Returns
PO Box 195
Bristol
BS20 1BT

Although never heard of any being fined for not returning it perhaps nobody reported that yet.