Ah yikes! Did they send your WHOLE PR application back or just your OWP application? I'm paying it now c:Here is link for payment instructions if you didn’t include the full payment.
http://www.cic.gc.ca/english/information/fees/fees.asp#temporarythe
http://www.cic.gc.ca/english/information/fees/openworkpermit.asp
It seems like their system has an issue and you have to manually add both fees. They sent my application back for non payment of the $155
Good Luck!