I had a similar situation with Avis years ago. I wrote a cover letter telling them that I traveled on business a lot and had some discretion as to which company I used and hinted that I could also influence co-workers. I enclosed a chack for what I considered to be reasonable reimbursement (it was about 80% of what they wanted) and never heard from them again. If you wanted to be really crafty, you could print an endorsement on the back that says, "Accepted as full settlement". (I'm not a lawyer so this is not legal advice.)