Can't buy postal money orders with a credit card (but can with a debit card):

http://www.usps.com/money/sendingmon...rs/welcome.htm

If there's a way to monetize them on a risk-free basis, it could definitely be a way to pick up a lot of miles / points quickly (or if one just has a big redemption coming up in the near future). However, I am not sure there's an easy way to monetize this.