wait...don't you need to mail the money with post or what ?
If you even bothered to check, you mail (not e-mail) money, a money order (still through mail), or use paypal. You'll have to provide your e-mail address (automatic through paypal) so the owner can send you your activation key, which you should share with
no one, unless you want your $20 to be wasted as your key will be deactivated.