I had this situation with an IPMO (International 
Postal Money Order), but I assume that it is the 
same for domestic money orders.

You go to the Post Office with your receipt and 
your half of the money order paperwork, and fill 
out a form.  Then you wait a while (90 days in 
the case of an IPMO), and then your money shows 
up in the mail as a check or MO from the USPS 
payable to you.

The downside is that you have to wait while the 
USPS convinces themselves that it is not going to 
be cashed, but the upside is that you can get 
your money back.