For my fellow musicians, here's what I've learned about mailing CDs for the least $. My CDs are Oasis Soft Spot 6 panel, which I chose for being as eco-friendly as possible while still looking top notch. It has very little plastic while still having a thick spine so that customers and radio stations can see your cd on the shelf.
The cheapest way to package this CD is in a simple 6x9 kraft envelope. They cost about 7 cents each:
For postage it's a good idea to use forever stamps, so that you don't have to get different stamps when the USPS raises rates. The first class letter rate is one "Non Machineable Surcharge" stamp which is 70 cents, plus one "Additional Ounce" stamp which is 21 cents. The total is 91 cents.
Note that this letter rate counts on the fact that the envelope is less than 1/4 inch thick. If the envelope is thicker it counts as a package and has a higher rate. Strangely, two CDs in the envelope counts as a package and costs over $3 to mail, so it is cheaper to send each CD in a separate envelope. 3 CDs won't fit in the envelope anyway.
For mailing to Europe, the same envelope can be used. It costs about $3 to send 1 or 2 CDs to Europe by International First Class mail. All mail goes by air now, so it is not too slow. Not bad!