Go to your local Post Office and buy a Postal Money Order. Costs about $1 if I recall correctly, is good as cash without the risk, because you have a receipt that can be tracked.
Never send cash through the mail.
Option B... Go to Wal-Mart and for about $9 you can buy a reloadable credit card. You can add cash to your card right there in the store and use it just like a normal credit card. Sure you have to pony up the extra few bucks to get it started, but then you can keep reloading it. It'll also be useful for making other internet purchases.
Now the silly answers may commence. Any post after this point is designed simply to amuse.