To legally use it after you have tried it, you must send me a postcard. I like receiving postcards from all over the world. Also, I think, that it is a nice way for the user to show appreciation for the program without becoming poor - sending a postcard is very cheap all over the world.
Many people seem to think that it is a joke.
But it is not. The only way of registering is to send me a real postcard which I can hold in my hands. Of course, you can also send other souvenirs or things from your country. You are not bound to it. Examples of this are my Delphi components .