They want you to fill in a W-9 so that they can issue you a 1099-MISC in January so that they can deduct the expense on their tax return. I would have them pay the auto repair shop directly and leave you out of the middle.
If they insist on paying only you and giving you a 1099 it is not income to you and you will need to report it on the return then simply make a negating entry to zero the income out. If this is what happens to you post back for more directions next year.
