They can cash the IPMO at their local post 
office. There is no fee to convert it to japanese 
yen,...as far as I know (I haven't heard any 
sellers complain to me after they receive them).

As far as conversion...when I get the price in 
yen, I use this site for the conversion: 
http://www.xe.net/ucc/

I think it's pretty close to what they will get 
in yen back. Hope this helps!