Hm. A form 1098-C is not required if the auto donation has a claimed value of less than $500, so I don't know why Turbotax would insist. Seems like a design error.
I would either,
1) mail the charity letter with the cover page. A letter is an adequate substitute for a form 1098-C if it contains all the information that would normally be on a 1098-C.
2) or, delete the car donation, and re-enter it as a "personal property" donation for the same value (less than $500). Since the form is not required, this should not be a compliance issue, and will remove the need to mail a form to the IRS.
