Doorman - Never Miss A Package Delivery Again

Recently @kane shared Parcel (http://fromparcel.com/ http://www.producthunt.co/posts/682) and ~month ago @ZackShapiro shared his new delivery startup, Luna (http://www.useluna.com/ http://www.producthunt.co/posts/77). Meanwhile, Amazon and Google are experimenting in same-day delivery.

I wonder how you differentiate a service like this? I noticed they're doing free delivery if you tweet or facebook share their service, so seems like they're hoping for WOM advantage.