Shipping a CD & cassette in a very small box is not be prohibitive. Just wrap the CD & cassette around some bubble wrap and put into box. You can buy small boxes at the post office that a CD & tape will fit into and it should not cost as much as $20.00 to ship it.

Just be sure you use proper mailing tape to secure box (scotch tape is not permitted and not secure enough anyway.)
I am also in California and have shipped CDs and tapes overseas and it did not cost $20.00!

You also have to get a declaration label (free) from the post office and declare the contents inside the box and the approx. value.
Doesn't have to be exact but just whatever you feel the value is (the cost of the CD and the cassette.)