I get asked what to do if you have a damaged or lost Netflix envelope, and here's the answer from the Netflix help system:
Q: I lost the prepaid return envelope used for returning the DVD.
A: If you misplaced one of our prepaid return mailers, you can mail up to two DVDs back in the same package. Or, you may send your DVDs to: P.O. Box 49021 San Jose, California 95161