See...

http://www.usps.com/ibu/postalfin/info/faq/infopos
t.htm

Also, the site contains a link to the detailed 
postal documentation ( 
http://pe.usps.gov/cpim/ftp/manuals/Imm/Immc3.pdf 
), where starting at the bottom of page 170 lays 
out all of the current rules on International 
Money Orders.

I didn't see anywhere cases where a post office 
can deny service, so you may want to discuss it 
further with the local Postmaster.

Chris