As far as PayPal are concerned, you would only need to provide proof of postage in the event of an Item Not Received claim, which I believe you get from most (if not all) couriers, so even if the buyer has these kinds of delivery instructions, you'd be unlikely to lose such a case.

In saying that, though, it doesn't mean things can't go wrong and cause problems for you, your buyer, or both. Perhaps, if you're really concerned about leaving the item at the property unsigned-for, you could contact your buyer and let them know with an order of this value, you'd be very concerned about a method of delivery that leaves the parcel vulnerable for any period of time, and would prefer a more direct delivery method (in other words, don't imply that you don't trust the buyer - I see no prominent reason to at this stage anyway - but that you have standard practices in place for all sales to ensure the utmost safety of every customer's purchase).