There's really no reason to send a postcard made of fabric and wire, but why the hell not? You can keep changing the shape of it and keep putting it back to the original rectangular shape. And of course you can mail the thing and your friends have a laugh when it shows up in their mailbox.
Here's the real postcard I made for my friends near Atlanta. I put a 39 cent stamp on it along with some glue for good measure and dropped it off in the mailbox yesterday morning. With luck, it should be there by the weekend.