General Atomics CP-1 Pigeon

The CP-1 Pigeon is a tiny drone meant to replicate the function of a carrier pigeon for personal mail. It is a small remote control aircraft with a gasoline engine and a large enough fuel tank to fly round-trip to anywhere in the continental US or Europe. The Pigeon is guided by a small onboard computer and the destination can be entered remotely through a smartphone app. After the letter is carefully rolled and placed into the payload compartment the Pigeon will fly to its destination maintaining 500 feet above the ground by means of GPS terrain awareness - high enough to avoid ground obstacles but low enough not to get sucked into a jet engine.

Upon arrival the aircraft will land itself if there is a large enough open area. If there isn't a large enough open area it will crash through Grandma's window and likely explode. After delivery of the message it is simply tossed back into the air and it returns to the owner.

Advantages:

Faster than an actual pigeon

Faster than the USPS

No stamps required

No training or pigeon-keeping required

Narrowly skirts FAA regulations regarding UAVs

The personal touch of a physical letter instead of email

UPDATE - I just got a second idea for an upgraded version of this, so I'll just post it here: the CP-2 Personal Falcon. The Falcon is the same vehicle as the pigeon but with upgraded software, a camera, and talons. It will follow you wherever you go, orbit above you (tracking your smartphone) and land on your shoulder when called using a combination of the tracking signal from your phone and the camera. The PF app can even send commands to attack one's foes, or catch small animals for dinner.