Can I send $12 through the mail, and it would arrive with the cash in there?

One opaque outer wrapper should should be sufficient to keep thieves hands off including any that might work for the Post Office. I know of no law about sending cash through the domestic mails but you might check with Postal authorities about sending large amounts of cash out of the country. a href="http://www.usps.com/welcome.htm" rel="nofollow"http://www.usps.com/welcome.htm/a