Western union money orders are what you need, buy as many as you want at a grocery store.
Western Union charges a $15 fee to get a refund on a money order, and it takes up to 30 days. Also IIRC these are limited to $500. My most recent cash in mail trade was for over $1000, so I would have had to buy three seperate money orders.
This isn't as great a deal as you're making it out to be.