Which would you rather do: pay $50 to overnight a package somewhere, or meet someone headed to that city who could drop off the package for you? Well, one costs $50 and the other gives you the opportunity to make a connection with another human.
Introducing FlyMe, or JetEx, or Airbound, or some other reasonably creative name that will fit easily into some sort of bubble font with clouds around it as a logo. There are two users of the site: people who have a package they need delivered, or a package-holder, and people who are going someplace with spare room in their luggage or carry-on, or courier. The package-holder goes to the website to find a courier going to the place they need their package delivered. They send the potential courier a message saying what they need delivered, where, when and any other details. The courier either accepts or rejects the mission. If the courier accepts the mission, the package-holder meets the courier, the package is handed off, and the courier goes on their trip to deliver the package to the appropriate person or place. Once delivery is confirmed, the original package-holder gives points or badges or a tip or a gift or a like or tweet to the courier, raising his courier-ninja rating on the website, and earning the respect from all the other couriers. Like couchsurfing, only with packages.
Pros: it uses the wasted space on planes, connects people with people, and relies on the trustworthiness of man.
Cons: it relies on the trustworthiness of man, a critical mass is needed in order to find not only the right destination but originating city, and coordinating a meet-up in some places – like Southeastern Michigan, might be difficult.