I should have mentioned that you do need to accept Ebay's AI recommendation on parcel size/weight. If you change it dyou are then responsible but I no longer bother worrying about it unless my parcel is over the max length (61cm I think).

It might be worth you reading through the Ebay help pages on Simple Delivery as there has been all sorts of rumours quoted as facts on these forums. Hopefully someone will add the link for you.