Should I respond to demand letter for reimbursing a business expense?

I recently resigned from a job. This was just prior to a business trip (company conference) where the company already paid for an airline ticket. The ticket was non-refundable.

I received a demand letter from the company asking me to pay back the cost of the airline ticket. This demand letter came from the HR representative of the company, not a lawyer for them. No policy was stated wherein this was something employees had to do, in terms of reimbursing what I would consider a business expense.

I have been debating whether to respond (politely) and simply ask for this policy since I am unable to find it in any materials I was sent as part of employment. But I have been told that perhaps responding to the demand at all is a bad idea, particularly given that it didn't come from a lawyer or legal firm.

I should note that I'm in Illinois but the company is based in Boston, Massachusetts.

I'm just curious if there's general advice on whether responding (again, politely) is the right thing to do.