Royal Mail / Parcel Farce deliver the standard delivery Amazon stuff, and this service doesn't require a signature for the goods. Whilst it is a retarded thing to do (leaving it on the doorstep I mean), I doubt you have a leg to stand on.
Always spec the Express Delivery for expensive / bulky goods, or get it delivered to your work address like I do, always someone in to sign for it then.
Not the answer you wanted to hear, but still argue with Amazon none the less.