You shouldn't, and nobody has suggested that you should. That is to say the compensation should match the actual inconvenience and expenses.
Mind you, this is all a bit of an irrelevance, since most correspondence of this kind need not involve recorded delivery. Ordinary post will suffice, and the receiving party won't know whether you've been to the PO to get a certificate of posting or not, and so you can save yourself the trip. Just walk to your nearest posting box.