PP guy told me the same thing in the very last coversation we had with them, regarding a proof of postage.

It was quite an interesting revelation, that being that if seller sends the item without tracking, they award the payment to the buyer because of claimed non receipt, and seller contests the decision - the seller will win as legally they did send it,  they can't be responsible for the parcel that the third party is handling (PO or a courier) - and the buyers can leave negative f/back, but that is all they can do.

But, saying that, all good sellers don't want to see anyone out of pocket and will replace or refund, - but they don't have to.

Maybe because of the f/back (ability to leave it) buyers and sellers have not tested this out, but if a seller refused to do anything at all for the buyer as they have already supplied it once, it really is up to the buyer to secure the postage by paying more.

I have often read advice on the boards given to everyone to not pay anything registered as that is for seller's protection, but in reality is other way around.

In the eyes of the law (not PP, as they are not the law, that is just one of their rules to make their job easier, applying only to Ebay, not all web site purchases), once buyer paid, seller posts - and that is the end of it.