It feels like given that Amazon give you no agency in when the package arrives, and also no guarantee of forewarning, then they've given you no option to prevent the package remaining on your doorstep, the liability should morally lie with them.

(Legally, could be more cloudy depending on jurisdiction, but I'm sure their tendency to say packages will require a signature on delivery but then to leave them on the doorstep with the driver's signature alone probably does them no favours)