You will cash it at your bank, wait a bit, think all is fine. They pick up, along with the $30. A few days later the bank tells you the cashiers check was fake. It generally takes the bank a bit to figure out it's fake. You could quite easily get someone to send you one of the fake cashiers check and see how long it takes for the bank to tell you its fake (assuming you don't have a problem giving one of these scammers your address). I've always been tempted to see what one of these brilliant forgeries look like except I don't want to give my address to those people.