Maybe I'm just being a bit dim, but it seems that many sites don't give you any idea of P&P costs until you have added to basket and gone through some of the purchasing process. Surely it can't be that difficult to include a "calculate postage" part of a website?
I'm on a site right now (I know leaving it very late for Christmas...) looking at a low-value item. If the postage if disproportionate to the cost of the item, then I'm not interested in buying it, but at no point have I been given any indication of P&P. I've added to basket and a message has come up saying "once you have clicked through to the next step your items will be picked, packaged and despatched... no orders can be deleted or amended after this point". Very helpful.