No never use cash.
I would if I were you get a paypal account for this stuff. Its quick, its easy and as long as you DONT send payment as a gift you can get your money back if need be.
Cash if it gets lost its gone, if mailman sees money and keeps it then its lost, if the person you decide wants to keep the cash and screw you then who is to say he ever got it or you sent it?
Money orders you have to pay, you have to go get one everytime you need one. Paypal in 30 seconds your golden and if you have a problem you can bitch to them, anyother way no one will care what happens.