Tom Donahue says: "I believe there is a USPS strong suggestion to avoid sending cash through the mail, but it has worked so far."
Actually, it's illegal to send cash of any denomination or currency out of the country, or so I've been told by USPS on a number of occassions.
Don't tell anyone, but just between me and y'all, I've done it once or twice, and never had a problem.
Duncan Granger
Lancaster, PA