You've done this before? That is about the most CARELESS thing you can do... there is absolutly NO proof of the payment once they get it.... unless you spend more money, and have 'verified postage' or whatever, where it has to be signed for, and even THEN... that just says that you sent a package, you have NO WAY of proving that you sent the cash.....
Ya, you really shouldn't be shopping online if you trust people that much. That is why we have Money Orders it is like sending cash and having proof of sending payment.....