The idea is simple, you buy a tag which you add to your keychain. Engraved on the tag is a unique number which is linked to your address. Now, if someone finds your keychain on the street the tag tells the person to throw it in the next postbox. Then the post just look up the address using the ID and mail the keychain back to you. All you have to pay is the postage fee. Beautiful idea and nicely executed. Would love to have the same service in Germany. The service provider has a website which can be found here.