First, in your garden pack up the pinata that needs cloning. Send it to yourself, when it says you got mail, press Y.

Here's the tricky part. Got to send crate, and as soon as you click send crate, press X rapidly.

If you did it right, it will send and the crate menu will come up.

Change the discription to anything, you can't leave it blank.

Then exit the crate menu, you have to exit the garden now, because the crate in your garden doesn't work.

Come back and now you have the crate in your garden working, and one in your post office. You could put CC, or pinatas in the garden.