Using my shop, i offer a discount of one postage cost for up to 5 cd's, however i have had many buyers which buy one cd a week and keep paying multiple pounds for posting.
I am sure that many other sellers offer rates which are not totally stood out for all to see, for all you buyers out there, email the seller, ask if theres any chance for postage discounts or if there is any available. Not all sellers specify this but will offer it.
Remember browse the shops as all links are sometimes hidden away and you can find these links to be highly cost effective, and for the sake of a quick cheeky email it is worth it. Us as a seller and a few people i have bought off will be quick to chuck in an offer, especially if that email stands out.
I know it sounds a lot and maybe a waste of time, but if you are a repeat buyer it can be cost effective! Remember just one quick email could save you a few pounds, its not much but it can be worthwhile.