according to my local post office, about a year ago the law or post office policy changed and it's no longer permitted to convert prepaid cards like this amex card, vanilla debit, etc., to money orders. something about money laundering and gaining points, rebates, etc.,this way. somebody put a stop to it. she didn't know who but she said she got stuck with three cards herself. if your name isn't printed on the card, their system won't accept it.
ymmv