AC @ 08:32
Wrote : "Personally, I put letters like that back in the post with a label over the address, but without postage. The idea is that only when it starts to cost something, companies will pay attention."
Can you explain that? You put a label over your own address? So is BT's address pre-printed on the back, and if so you are assuming it will go back to them and they be charged for the return postage?
I am not sure it will work like that. As long as you really are at your address, I believe that *you* could be charged for the return postage because the Post Office have already done their job properly.
Of course, if BT were to use a pre-paid return envelope, use that (but they don't work that way, being a phone company). I send loads of mailshots like that back, often adding more stuff to get the weight into the next cost bracket. Depending on context I remove anything to identify myself.
I also make a note of mail spammers "Freepost" addresses - you can use any envelope for those. I have a large collection of unused pre-addressed envelopes (they come with electricity bills etc but I pay on-line) and I once sent about a fifty of them back to a mail spammer using labels I printed myself with their Freepost address. I am not sure how far you can go with this - could I put one on a parcel with a few house bricks in it, does anyone know?