Try sending another PFD letter, (CMRRR), with a bit stronger language. Stress you will pay "quickly" to resolve the matter to mutual satisfaction. Did you say you would pay the full amount, or partial? Also, how long has it been since PFD was sent?
P.S. If you receive no response to a 2nd letter, you may want to call and speak to someone. PFDs are not "allowable" per industry standards, so understand that you are asking them to "document" a nonallowable practice.
