All you need is take a pair of boxes down that are large enough for anything that might be delivered, bolt them down to anchors in concrete or the wood on your porch, and put instructions on them for any deliveryman to drop the parcel, and lock the provided padlock. Unless you get more than two deliveries a day, you should be in good shape, and no worrying about mechanical problems. (I have a heavy duty trunk drilled to do this exact thing, but I haven't gotten to drill the concrete anchors yet.)