I bought something which the seller charged£6.95 to post and it cost 74p in a plain envelope (actually it was just a folded bit of brown paper)
I've asked very politely for a refund and said that I understood it wasn't possible to know previous to posting how much it cost to post. They could have sent it in a box and they could have added recorded delivery and it would have cost £3.80 second class to do so - but they didn't do any of that.