One of the holy grails of all frequent flyers has always been finding a way to use a mileage-earning credit card to essentially "buy cash" with no, or truly minimal fees. (A la...Presidential Dollar Coins)
The United States Postal Service though does not allow purchases of money orders with CREDIT cards, only debit ("pin-based") cards.
However I believe there ARE still a few debit cards that do earn frequent flyer miles.
You should easily be able to find more information about those cards by searching the Flyertalk boards.