Hi Reki!
First of all it's a nice thought of you to send something to your penpal. I had a penpal from Japan before and was extatic when she sent me surprises with her letters, so I'm sure you'll make your penpal happy.
But then, just a reminder/warning - be extra careful when sending goods abroad, i.e. do check the regulations regarding food especially. It's sometimes forbidden to send some edible things, seeds, wood etc. to some countries, and if I remember right, with Japan you'd better check that... You can search online or they'll prolly know at your post office.